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,
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,
问题解答：
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