答案 A to run.see是感官动词,主动句里用于“sb saw sb do sth"时,宾补do sth省略to,但变成被动句时要加上to.在感官动词（see,hear,feel,listen to ,notice,watch,observe等）和使役动词（let,have,make ）后的补足语中,不定式不带to.但这类句子变成被动语态时,必须带to.
I saw him play in the park.
→He was seen to play in the park.