As it turned out to be a small house party, we so formally.
A. needn’t dress up
B. didn’t have to dress up
C. might not have dressed up
D. needn’t have dressed up
网友回答
A. needn’t dress up
B. didn’t have to dress up
C. might not have dressed up
(答案→)D. needn’t have dressed up
解析：根据前一句说这是一个小型的家庭聚会，所以后一句意思是：本没必要，故选D。