21. The education of _______ young is always _______ hot and serious topic in modern society.
A. the；/ B. the；a C. /；the D. a；the
22. —You didn’t wait for Mr. Smith last night, did you?
—Yes, but we ________. He didn’t return home at all.
A. don’t have to B. needn’t have C. didn’t need to D. should have
