20.Many of them turned a deaf ear to his advice,　　 they knew it to be valuable.

A. even though　　 B.now that　　 C. as if　　 D.so that

19.With the temperature　　　 so rapidly, we couldn’t go on with the experiment.

　 A. fall　　　 B. fell　　　 C. falling　　 D. fallen

18.How did it　　　 that he knew where we were？

　 A.bring about　　 B.bring in　　e about　　e across