15.口,3a-b,2a,a+b
(A)4a+2b
(B)4a-2b
(C)2a-b
(D)5a-2b
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統計: A(6), B(30), C(2), D(0), E(0) #2717665
統計: A(6), B(30), C(2), D(0), E(0) #2717665