we----- get there,when it ------ to rain.a were about to get

A.were about to get; began
前面用进行时态或者正要发生某事,后面用when引导从句,表示 “当.时,.”或者“.,就在这时,.”
如：I was washing the dishes when he came.我正在洗盘子,就在这时,他来了.（当他来的时候我正在洗盘子）
He was about to go out when his daughter cried.他正要出门,就在这时,他女儿哭了起来.