They spend two hours on their work.保持句意不变 They spend two hours ____their work.

问题描述:

They spend two hours on their work.保持句意不变 They spend two hours ____their work.

spend 时间 on sth.花时间在某事上 =
spend 时间 in doing sth.花时间做某事 ,这个结构里in可以省略
所以一个空格填写:finishing