Tom's school was on the other side of the road, so he was told to watch out for cars when ______ the street.

【正确答案】 A

【答案解析】 当状语从句的主语和主句的主语一致，且从句中含有be动词时，可以省略状语从句中的主语和be动词，即题中从句中的crossing为he was crossing的省略。故选A。