题目内容：

a car accident happened on my way home yesterday 改为同义句

优质解答

=A car accident took place on my way home yesterday
happen = take place 发生

追问：

when the ufo arrived,i______ 1.am walking 2.was walking 3.walked 4.walking

追答：

选 was walking 用过去进行时与前面的过去式 arrived 对应、