第二部分：英语知识运用（共两节，满分45分）
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。
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