单项填空
21.Go back home now，Mary. Your parent your safety.
A. are concerned about B. are concerned with
C. are concerned in D. concern of
22.—Do you know the most ancient city in China—Xian well?
—No，this is the first time I here.
A. came B. have been C. am coming D. come
23. I arrived half an hour earlier I had time to talk with him before the meeting.
A. as soon as B. as a result C. in order to D. so that
24.As I there，my friend had left three days .
