题目内容：

Look There are many people ____ ____ their bikes on the street.
Look There are many people _____ _____ their bikes on the street.

优质解答

这句话是用现在分词作定语的结构,因为句子里面已经有了一个be动词,后面那个“骑车”只能用非谓语动词的结构,因为“骑车”和“人”之间是主动关系（人可以做出骑车的动作）,所以用现在分词,表示主动的意义,这一格要填riding on.希望我能帮助你解疑释惑.