need做实词和情态动词有什么区别

问题描述:

need做实词和情态动词有什么区别
The students were very glad that they_________their papers last weekend as the teacher was away on vacation.
C.needn't have handed in D.didn't need to hand in
D .
为什么选 C 不行?

选C的话就是虚拟语气了