need在这道选择题中到底怎么选?

题目

need在这道选择题中到底怎么选?
- Well,it's a pity I didn't buy that recently published dictionary yesterday.
- ____.I have already bout one for you.
A.You needn't buy it.
B.You needn't have bought it.
C.You didn't need to buy it.
D.You don't have to buy it.
A里面可以看作need做情态动词用,C里need做行为动词用,D用法也对,可是答案只有C是正确的,

答案

C.
A.在这种情况下不能直接用need.
B.整个句子的grammar就有错.
C.didnt:代表过去式.正确
d.dont':应该是过去式