16.He would have given you more help， if he ________ so busy.

　　A. have not been

　　B. had not been

　　C. not have been

　　D. not had been

　　17.After months of voyage， Columbus arrived in ______ later proved to be a new continent.

　　A. where

　　B. which

　　C. what

　　D. that

　　18.I was really anxious about you. You ______ home without a word.

　　A. mustn‘t leave

　　B. shouldn’t have left

　　C. couldn‘t have left

　　D. needn’t leave

　　19.He was ______ to withdraw from running for the presidency.

　　A. trusted

　　B. credited

　　C. convinced

　　D. believed

　　20.While _____ the book， he nodded from time to time

　　A. reading

　　B. read

　　C. is reading

　　D. is read