下列式子正确的是 () ←-|||-A、 lim _(xarrow 0)dfrac (xy)({x)^2+(y)^2}=0 B、 lim _(xarrow 0)dfrac (xy)(x+y)=0 C、 lim _(xarrow 0)dfrac (x{y)^3}({x)^2+(y)^6}=0 D、 lim _(xarrow infty )dfrac ({x)^2+(y)^2}({x)^4+(y)^4}=0