■Have the problems they have_____ been solved.A concentrated B,concentrating C concentrated on D.concentrating on
问题描述：
■Have the problems they have_____ been solved.
A concentrated B,concentrating C concentrated on D.concentrating on
答
C concentrated on
答
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
答
选C.concentrated on,专心于..
肯定句:The problems they have concentrated on have been solved.
答
C
concentrate on 集中力量做某事
Have the problems they have_____ been solved。（they have修饰the problems ）