Scarcely had she put down the receiver —— the telephone rang again 答案选的when但是为什么不能选then?

Scarcely had she put down the receiver —— the telephone rang again 答案选的when但是为什么不能选then?
Scarcely had she put down the receiver —— the telephone rang again 答案选的when但是为什么不能选
then?

Scarcely had she put down the receiver —— the telephone rang again 答案选的when但是为什么不能选then?
解析：可以说这是固定搭配
had scarcely / hardly...when...或had no sooner...than...一.就.
只不过是句中把scarcely 提前,构成倒装结构
翻译：她刚一把接到的物品放下,电话就又打来了.

scarcely +完成时（还必须用倒装句）when +现在时 此为一个固定用法

因为这是时间状语从句