选择：My mather is busy homework.A.doB.doesC.doingD.to do为什么?

选择：My mather is busy homework.
A.do
B.does
C.doing
D.to do
为什么?

C.doing
因为：be busy doing sth：忙于做某事

C.doing
be busy doing固定短语，“忙于。。。”