第一节：单项选择。（共10小题；每小题1分，计10分）
从每小题所给的A、B、C、D四个选项中选出可以填入空白处的最佳答案。
21.---What was Marc doing when his mother got home yesterday ?
----He was playing _____________ guitar.
A. the B. a C. an D. /
22.---When did the great earthquake in Wenchuan happen ?
---It happened __________2:28 __________the afternoon of May 12, 2008.
A. on, in B. at, on C. at, in D. on, on
