I've never seen anyone run so fast--________ David go.

A.just watch

B.just to watch

C.just watching

D.just having watched

答案及解析： A

解析：

破折号表示解释说明，故后一句话就是前一句话的补充，应为两个句子。祈使句的基本结构为do sth. 或don't do sth. 。