1.设 (xy,dfrac (x)(y))=((x+y))^2 则 f(x,y)= ()-|||-(A) ^2((y+dfrac {1)(y))}^2. (B) dfrac (x)(y)((y+1))^2-|||-(C) ^2((x+dfrac {1)(x))}^2. (D) dfrac (y)(x)((y+1))^2