4.设区域D为 ^2+(y)^2leqslant (R)^2, 则iint (dfrac ({x)^2}({a)^2}+dfrac ({y)^2}({b)^2})dxdy 等于 ()-|||-(A) pi (R)^4(dfrac (1)({a)^2}+dfrac (1)({b)^2}) (B) dfrac (pi {R)^4}(2)(dfrac (1)({a)^2}+dfrac (1)({b)^2})-|||-(C) dfrac (pi {R)^4}(3)(dfrac (1)({a)^2}+dfrac (1)({b)^2}) (D) dfrac (pi {R)^4}(4)(dfrac (1)({a)^2}+dfrac (1)({b)^2})