They are busy _ this week.They _ very tired.Why not _ a rest?

题目

They are busy _ this week.They _ very tired.Why not _ a rest?
A.work,may be,to have B.to work,can be,have C.working,nust be,have
D.with work,will be,having

答案

C.working,must be,have
be busy doing sth忙于做某事
must be表肯定推测
why not do sth为什么不?