The research mainly deals with the difficulty the students have their attention pm their study all the time.
A. to focus
B. to be focused
C. focusing
D. focused
网友回答
A. to focus
B. to be focused
(答案→)C. focusing
D. focused
解析：have difficulty (in)doing sth在做某事方面有困难 the students have…为定语从句，省略了连接词that.故选C