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下列等式成立的是
A.am(am-a2+7)=a-a2+m+7a
B.am(am-a2+7)=amm-a2m+7am
C.am(am-a2+7)=a2m-a2+m+7am
D.am(am-a2+7)=a-a2+m+7am