满足A∪B=｛a1,a2｝的集合A,B的组数为

题目

满足A∪B=｛a1,a2｝的集合A,B的组数为
A.5 B.7 C.9 D.10

答案

C
若A为空集,则B为{a1,a2}
若A为{a1},则B可为{a2},{a1,a2}
若A为{a2},则B可为{a1},{a1,a2}
若A为{a1,a2},则B可为空集,{a1},{a2},{a1,a2}