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1. It must be pointed out that it is one of our basic State policies to control population growth while raising the quality of the population. 2. It is（has been）estimated that the worlds population could reach 6 billion by the end of the century. 3. It must be kept in mind that there is no secret of success but hard work. 4. It can be seen from this that there is no difficulty in the world we can overcome. 5. As is known man is the product of labour. 6. It is a common saying that man who has a settled purpose will surely succeed. 7. It is clear that the enemy has no desire for peace. 8. It is hard to imagine how Edison managed to work twenty hours each day. 9. Its hard to say whether the plan is practicable. 10. There is no doubt that you will be helped by others if you have any difficulties. 11. It seems certain that they have made a series of experiments. 12. It is said that bats have been using radar for millions of years. 13. To tell the truth，many mistakes we made could have been avoided. 14. As we know，it was not until recently that the problem was solved. 15. It must be admitted that you havent done what you promised to do. 16. In my opinion，this computer is different from that one you saw. | 
