The research mainly deals with the difficulty the students have ______ their

题目

The research mainly deals with the difficulty the students have ______ their
attention on their study all thetime.
A.tofocus B.to be focused C.focusing D.focused
为什么

答案

the students have是前面省略了that的定语从句,修饰the difficulty 的,去掉不看,就是：
The research mainly deals with the difficulty focusing attention on their study all thetime
difficulty和focus是主动关系,用现在分词
这份研究主要是解决学生们在学习中（能否）专心的困难