I've never seen anyone run so fast--________ David go.
A.just watchB.just to watchC.just watchingD.just having watched在线课程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. 。