21. You shouldn’t accept a gift from a person _____ you don’t know him.
A. until B. unless C. if D. whether
22. Attention, please. The plane to London ______ off at 9: 00 pm.
A. take B. takes C. took D. has taken
23. If it ______ tomorrow, we’ll have a picnic in the park.
A. isn’t rain B. won’t rain C. don’t rain D. doesn’t rain
