21. —It really shocked us that he should have made such_______ rapid progress in his study in such a short time!
—No surprising. He has been making _______ most of his spare time to study.
A. a; the B. a; / C. /; the D. /; /
22. It is strongly recommended the machines __________ every other month.
A. to check B. checking C. checked D. be checked
23. No one knows exactly ________ this important discovery was made.
A. that B. what C. that how D. when
