21. We know that ______ play, often seen as an activity of kids, is _______crucial part of healthy child development.
A. the ; a B. / ; the C. / ; a D. a; a
22. To find the missing old man, all the relatives _______what they can in the last six hours.
A. did B. do C. had done D. have been doing
23. I really want to buy the girl a gift on her birthday, _______ she would like to accept.
A. one B. it C. that D. the one
