■Have the problems they have_____ been solved.

来源：百度知道 编辑：UC知道 时间：2024/06/27 08:18:36
A concentrated B, concentrating C concentrated on D. concentrating on

选C.concentrated on，专心于..
肯定句: The problems they have concentrated on have been solved.

C
concentrate on 集中力量做某事
Have the problems they have_____ been solved。（they have修饰the problems ）

C 这里应该是一个问句吧 意思是：
他们专心于的问题解决了吗？
concentrate on 是一个固定搭配，on 不能省略

C concentrated on