gre考试阅读真题解析(B)
<p> <span word="Bracken">Bracken</span> <span word="fern">fern</span> <span word="has">has</span> <span word="been">been</span> <span word="spreading">spreading</span> <span word="from">from</span> <span word="its">its</span> <span word="woodland">woodland</span> <span word="strongholds">strongholds</span> <span word="for">for</span> <span word="centuries">centuries</span>, <span word="but">but</span> <span word="the">the</span> <span word="rate">rate</span> <span word="of">of</span> <span word="encroachment">encroachment</span> <span word="into">into</span> <span word="open">open</span> <span word="countryside">countryside</span> <span word="has">has</span> <span word="lately">lately</span> <span word="increased">increased</span> <span word="alarmingly">alarmingly</span> <span word="throughout">throughout</span> <span word="northern">northern</span> <span word="and">and</span> <span word="western">western</span> <span word="Britain">Britain</span>. <span word="A">A</span> <span word="tough">tough</span> <span word="competitor">competitor</span>, <span word="bracken">bracken</span> <span word="reduces">reduces</span> <span word="the">the</span> <span word="value">value</span> <span word="of">of</span> <span word="grazing">grazing</span> <span word="land">land</span> <span word="by">by</span> <span word="crowding">crowding</span> <span word="out">out</span> <span word="other">other</span> <span word="vegetation">vegetation</span>. <span word="The">The</span> <span word="fern">fern</span> <span word="is">is</span> <span word="itself">itself</span> <span word="poisonous">poisonous</span> <span word="to">to</span> <span word="livestock">livestock</span>, <span word="and">and</span> <span word="also">also</span> <span word="encourages">encourages</span> <span word="proliferation">proliferation</span> <span word="of">of</span> <span word="sheep">sheep</span> <span word="ticks">ticks</span>, <span word="which">which</span> <span word="not">not</span> <span word="only">only</span> <span word="attack">attack</span> <span word="sheep">sheep</span> <span word="but">but</span> <span word="also">also</span> <span word="transmit">transmit</span> <span word="diseases">diseases</span>. <span word="No">No</span> <span word="less">less</span> <span word="important">important</span> <span word="to">to</span> <span word="some">some</span> <span word="people">people</span> <span word="are">are</span> <span word="brackens">brackens</span> <span word="effects">effects</span> <span word="on">on</span> <span word="threatened">threatened</span> <span word="habitats">habitats</span> <span word="and">and</span> <span word="on">on</span> <span word="the">the</span> <span word="use">use</span> <span word="of">of</span> <span word="uplands">uplands</span> <span word="for">for</span> <span word="recreational">recreational</span> <span word="purposes">purposes</span>, <span word="even">even</span> <span word="though">though</span> <span word="many">many</span> <span word="appreciate">appreciate</span> <span word="its">its</span> <span word="beauty">beauty</span>.</p><p> <span word="Biological">Biological</span> <span word="controls">controls</span> <span word="may">may</span> <span word="be">be</span> <span word="the">the</span> <span word="only">only</span> <span word="economic">economic</span> <span word="solution">solution</span>. <span word="One">One</span> <span word="potentially">potentially</span> <span word="cheap">cheap</span> <span word="and">and</span> <span word="self">self</span>-<span word="sustaining">sustaining</span> <span word="method">method</span> <span word="of">of</span> <span word="halting">halting</span> <span word="the">the</span> <span word="spread">spread</span> <span word="of">of</span> <span word="bracken">bracken</span> <span word="is">is</span> <span word="to">to</span> <span word="introduce">introduce</span> <span word="natural">natural</span> <span word="enemies">enemies</span> <span word="of">of</span> <span word="the">the</span> <span word="plant">plant</span>. <span word="Initially">Initially</span> <span word="unrestrained">unrestrained</span> <span word="by">by</span> <span word="predators">predators</span> <span word="of">of</span> <span word="their">their</span> <span word="own">own</span>, <span word="foreign">foreign</span> <span word="predators">predators</span> <span word="are">are</span> <span word="likely">likely</span> <span word="to">to</span> <span word="be">be</span> <span word="able">able</span> <span word="to">to</span> <span word="multiply">multiply</span> <span word="rapidly">rapidly</span> <span word="and">and</span> <span word="overwhelm">overwhelm</span> <span word="intended">intended</span> <span word="targets">targets</span>. <span word="Because">Because</span> <span word="bracken">bracken</span> <span word="occurs">occurs</span> <span word="throughout">throughout</span> <span word="the">the</span> <span word="world">world</span>, <span word="there">there</span> <span word="is">is</span> <span word="plenty">plenty</span> <span word="of">of</span> <span word="scope">scope</span> <span word="for">for</span> <span word="this">this</span> <span word="approach">approach</span>. <span word="Two">Two</span> <span word="candidates">candidates</span>, <span word="both">both</span> <span word="moths">moths</span> <span word="from">from</span> <span word="the">the</span> <span word="Southern">Southern</span> <span word="Hemisphere">Hemisphere</span>, <span word="are">are</span> <span word="now">now</span> <span word="being">being</span> <span word="studied">studied</span>.</p><p> <span word="Of">Of</span> <span word="course">course</span>, <span word="biological">biological</span> <span word="control">control</span> <span word="agents">agents</span> <span word="can">can</span> <span word="safely">safely</span> <span word="be">be</span> <span word="released">released</span> <span word="only">only</span> <span word="if">if</span> <span word="it">it</span> <span word="can">can</span> <span word="be">be</span> <span word="verified">verified</span> <span word="that">that</span> <span word="they">they</span> <span word="feed">feed</span> <span word="solely">solely</span> <span word="on">on</span> <span word="the">the</span> <span word="target">target</span> <span word="weed">weed</span>. <span word="The">The</span> <span word="screening">screening</span> <span word="tests">tests</span> <span word="have">have</span> <span word="so">so</span> <span word="far">far</span> <span word="been">been</span> <span word="fraught">fraught</span> <span word="with">with</span> <span word="difficulties">difficulties</span>. <span word="The">The</span> <span word="first">first</span> <span word="large">large</span> <span word="shipment">shipment</span> <span word="of">of</span> <span word="moths">moths</span> <span word="succumbed">succumbed</span> <span word="to">to</span> <span word="a">a</span> <span word="disease">disease</span>. <span word="Growing">Growing</span> <span word="enough">enough</span> <span word="bracken">bracken</span> <span word="indoors">indoors</span> <span word="is">is</span> <span word="difficult">difficult</span>, <span word="and">and</span> <span word="the">the</span> <span word="moths">moths</span> <span word="do">do</span> <span word="not">not</span> <span word="readily">readily</span> <span word="exploit">exploit</span> <span word="cut">cut</span> <span word="stems">stems</span>. <span word="These">These</span> <span word="are">are</span> <span word="common">common</span> <span word="problems">problems</span> <span word="with">with</span> <span word="rearing">rearing</span> <span word="insects">insects</span> <span word="for">for</span> <span word="biological">biological</span> <span word="control">control</span>.</p>
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