Millie always wears a special costume------ a mask at the pa

Millie always wears a special costume______ a mask at the party.
Millie always wears a special costume______ a mask at the party.
A.with B.on C.for D.in 应该选什么?给的答案是B,可是为什么不选A啊?我要恨确定的答案哦.