21. Lawrence Craven, a doctor from the USA, is the author of several published reports, one of _______ introduced the idea in 1953 _______ aspirin could reduce the risk of heart attacks.
A. them; when B. which; that C. what; that D. which; when
22. --- Really? Who will give _________ lecture?
---- Mr. Wang, ________ professor from Nanjing University.
A. the; 不填 B. a; the C. the; a D. a; a
23. My sister met him at the Grand Theatre yesterday afternoon, so he _______ your lecture.
A. couldn't attend B. mustn’t have attended
C. shouldn't attend D. couldn’t have attended
