第一节：语法和词汇知识（共15 小题；每小题1分，满分15分）
从A、B、C、D四个选项中，选出可以填入空白处的最佳选项。
例: We ______ last night, but we went to the concert instead.
A. must have studied B. might study
C. should have studied D. would study
答案是C。
21. Unfortunately, when I dropped in, Doctor Li _____ for Beijing to join in the fight against SARS, so we had not enough time for a talk.
A. just left B. has just left
C. is just leaving D. was just leaving
