It happened when I____pass the museum A walk B am walking C will walk D was walking 选哪个,为什么

问题描述:

It happened when I____pass the museum A walk B am walking C will walk D was walking 选哪个,为什么
可是when后面不是跟过去式吗

D 前面是过去式 而且强调的是进行中.因而是过去进行时