从A、B、C、D四个选项中，选出可以填入空白处的最佳选项。
21. What ______ public is concerned about is whether medical workers and scientists will be able to find out___ cure for this new disease in a short time.
A.不填;不填 B.不填;the C. the;不填 D. the; a
22._____ good, these cookies sell well.
A. Smelled B. Smelling C. Being smelled D. To smell
23.—Haven't I told you that I like the beef well done?
—Yes, but I _____ it longer than I usually do.
A. was cooking B. will cook C. cooked D. had cooked
24. We' ve published large quantities of books. This year___ we've published three million.
A. only B. just C. alone D. merely
