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③It has been ________ ________ for more than 500 years. 它已经存在500多年了。 ④Nowadays, there are still many people wondering when the earth ________ ________ ________. 现在仍有很多人想知道，地球是何时产生的。 答案：①exist on ②There existed ③in existence ④came into existence 3．puzzle n．迷；难题 vi & vt. (使)迷惑；(使)为难 归纳拓展 puzzle about/over 苦苦思索；仔细琢磨 puzzle sth. out 琢磨出……的答案；开动脑筋 be puzzled by 被……迷惑 be in a puzzle about sth. 对某事感到不解 set a puzzle for sb. (＝set sb. a puzzle) 出个谜语让某人猜 be in a puzzle 不知如何是好 puzzled adj. 感到困惑的 puzzling adj. 令人困惑的 ③Where the missing plane has gone still remains a puzzle ________ us. 失联飞机去了哪儿对我们来说仍是个谜。 ④There was a ________(puzzle) look on the little girl's face because a ________(puzzle) problem ________(puzzle) her. 在这个小女孩的脸上带着迷惑的表情，因为一道令人迷惑的问题让她感到困惑。 答案：①puzzled ②about/over ③to ④puzzled; puzzling; puzzled 4．pull n. &vt. 拉(力)；托；牵引力 归纳拓展 pull through 康复；痊愈 pull back 拉回来；缩回；(使)后退；撤退 pull down 拉下(窗帘等)；拆毁；拆掉(房屋等) pull out 拔出；(列车等)出站；启程 pull in (列车等)驶进；到站；(船)靠岸 pull together 齐心协力；团结起来 pull sb. through (使)渡过危机(难关) pull ... up 把……往上拽；停止 pull over (机动车辆等)停靠(在路边) | 
