20. After a thorough search, the rescue team finally brought the mountain climbers back, ______.

　 A. safe and sound　　　　　　　　　　　　　　 B. safely and sound

　 C. safely and soundly　　　　　　　　　　　　　　　 D. safe and soundly

19. Experts are trying to work out a plan to deal with the problems caused by global warming ______ it is too late.

　 A. when　　　　　　　　　 B. if　　　　　　　　　　　 C. before　　　　　　　　　 D. until

18. Teachers prefer to measure our performance ______ the efforts we make rather than the intelligence we have.

　 A. in addition to　　　　　 B. in terms of　　　　　　　 C. in honor of　　　　　　 D. in response to