They spent half an hour (pack)their bags 用适当形式填空 过去完成时
网友回答
They spent half an hour packing their bags.
spend time (in) doing sth 是固定结构（in往往省略）,这里不用过去完成时.
过去完成时用来表示“在过去的某个动作之前已完成的动作,也就是”过去的过去”.
When we arrived at the station, the train had already left.
======以下答案可供参考======
供参考答案1:
They spent half an hour (packing)their bags
sb spend (times...)in doing sth 或sb spend sth on doing sth
供参考答案2:
packing