21. Some researchers are finding that daydreaming may be important to ______ mental health and it is ______ good means of relaxation.
A. the; 不填 B. 不填 ; the C. 不填; a D. 不填; 不填
22. I know one of the things I was unsure about was _____ hard it would be to climb this mountain.
A. what B. that C. however D. how
23. She told me that she would not go with us if it ____ the next day.
A. was raining B. rains C. rained D. had rained
