单选题 Did you see them ___________ in the river just now?
A. to swim B. swam C. swimming

【正确答案】 C

【答案解析】see sb doing sth看见某人在干某事。还可以说see sb do sth，其中不定式符号to被省去了。类似的用法还有：watch sb doing sth，find sb doing sth等。