33.若f(x)為x之m次多項式,g(x)為x之n次多項式,則下列何者正確? (A)f(x)+g(x)的次數為m+n(B)f(x)+g(x)的次數為mn (C)f(x)·g(x)的次數為mn(D)f(x).g(x)的次數m+n