单项选择：
21. Historically, ______ main material for making tables has been wood, but ______ metal and stone have also been used.
A. the; / B. /; / C. the; the D. /; the
22.Good news! We didn’t spend ______ we had expected.
A. as half much money as B. much money as half as
C. as much money as half D. half as much money as
23. What ______ during a person’s early years may affect the rest of his life.
A. happened B. has happened C. happens D. happening
24. As we all know, it was ______ that resulted in the terrible car accident.
