As it turned out to be a small house party, we so formally.
A. needn’t dress up
B. did not need have dressed up
C. did not need to dress up
D. needn’t have dressed up
网友回答
A. needn’t dress up
B. did not need have dressed up
C. did not need to dress up
(答案→)D. needn’t have dressed up
解析：考查情态动词和虚拟语气。句意：结果是一个小的家庭聚会，我们本可不必穿得这么正式。B项中的need不是情态动词；needn’t have done本不必做某事（实际上却做了）。故选D。