1. it is (has been) estimated that the world’s population could reach 6 billion by the end of the century.
　　 2. 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.
　　 3. upon hearing the unexpected news, he was so surprised that he couldn’t utter a word.
　　 4. it’s hard to say whether the plan is practicable.
　　 5. it must be kept in mind that there is no secret of success but hard work.
　　 6. as the saying goes, nothing in the world is difficult for one who sets his mind to it.
　　 7. as is known man is the product of labour.
　　 8. it can be seen from this that there is no difficulty in the world we can overcome.
　　 9. it seems certain that they have made a series of experiments.
　　 10. it must be admitted that you haven’t done what you promised to do.
　　 11. it is certain that we have a long way to go.
　　 12. it is clear that the enemy has no desire for peace.
　　 13. it is hard to imagine how edison managed to work twenty hours each day.
　　 14. there is no doubt that you will be helped by others if you have any difficulties.
　　 15. it is said that bats have been using radar for millions of years.