2. It is (has been) estimated that the world's 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. It's 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.