I think the police _____ the crime, so we will have the result s

I think the police _____ the crime, so we will have the result soon.
A、is busy investigating
B、is busy to investigate
C、are busy to investigate
D、are busy investigating
【正确答案】：D
【题目解析】：be busy doing sth.忙于干某事；the police是集体名词，谓语动词要用复数。