题目内容：

_the students walked along the street ,they sang happy A.AS B.FOR C.WHEN D.SINCE

优质解答

选C when
When the students walked along the street,they sang happy.
当学生们沿着街道行走的时候,他们高兴的唱着歌.
这是一个从句.而as 和for 均不能作为从句的引导词,所以排除A,B ,而since在这里意思不符合.只能用when,意思是：当……的时候.