It _____________ when they left the station.

[    ]

A. has rained　　B. is raining

C. rains　　  D. was raining

从所给四个选项中选择最佳答案

It ______ when they left the station.

[　　]

A．has rained　　　B．is raining

C．rains　　　　　D．was raining