21. His experiment done in ______ summer of 1970 was ______ failure.
A. /; a B. /; / C. the; a D. the; /
22. That passenger was very impolite to the conductor, ______, of course, made things even worse.
A. who B. which C. what D. that
23. —How would you like your coffee?
—_______.
A. It's well done B. Very nice, thank you
C. One cup. That's enough D. The stronger, the better
