【正文】
15. It is said that bats have been using radar for millions of years. 16. To tell the truth, many mistakes we made could have been avoided. 17. As we know, it was not until recently that the problem was solved. 18. In my opinion, this puter is different from that one you saw. 19. All this shows that nothing can prevent us from reaching our aims. 20. As far as we know, it took him more than a year to write the book. 21. It has been proved that every substance, no matter what it is, made up of atoms. 22. To be frank, whether you like it or not, you have no other choice. 23. It has been decided that we are going to build a railway whose base must be pl