It is the greatest happiness on earth_____A.love and be lovedB.loving and being lovedC.to love and to be lovedD.loving and loved应该选C,为什么不能选ABD,给理由,
问题描述：
It is the greatest happiness on earth_____
A.love and be loved
B.loving and being loved
C.to love and to be loved
D.loving and loved
应该选C,为什么不能选ABD,给理由,
答
C. to love and to be loved，不定式作真正主语，比较常用
答
C. to love and to be loved
这是不定式的主动和被动形式,做定语,修饰前面的happiness的