--- crossing the street ,he was knocked down by a car.选项:a、B

While crossing the street ,he - -- - - -
答案是while,不是during.你的句子是分词结构,可以改写成：while he was crossing the street,he - - -- - -
在分词结构里面,你的句子是while+现在分词.记住,during+名词短语.所以B是错误的选项.
又如
While( he was) staying in Shanghai,he visited me
===During his stay in Shanghai,he visited me
记住：while+ 现在分词,during+名词短语