21. —Is there anyone who is going to the Great Wall?
—________.
A. None B. No one C. No D. Not any
22._________a dark night, they took the sick boy to the hospital. And after an hour's curing, the boy was out of danger.
A. With B. At C. On D. In
23. —Did you remember to give Tom the key to the bike?
—Yes. I gave it to him _____________I saw him.
A. while B. once C. suddenly D. the moment