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 haven't done what you promised to do。

　　16. In my opinion, this computer is different from that one you saw。

　　17. It is certain that we have a long way to go。

　　18. All this shows that nothing can prevent us from reaching our aims。

　　19. As far as we know, it took him more than a year to write the book。

　　20. It has been proved that every substance, no matter what it is, is made up of atoms.