3.当表达式Pdx+Qdy中函数P,Q取（ ）时，此式在其定义域内必为某一函数的全微分.A. P=(-y)/(x^2)+y^(2), Q=(x)/(x^2)+y^(2)B. P=(y)/(x^2)+y^(2), Q=(x)/(x^2)+y^(2)C. P=(x)/(x^2)+y^(2), Q=(-y)/(x^2)+y^(2)D. P=(x)/(x^2)+y^(2), Q=(y)/(x^2)+y^(2)

A. $P=\frac{-y}{x^{2}+y^{2}}$, $Q=\frac{x}{x^{2}+y^{2}}$

B. $P=\frac{y}{x^{2}+y^{2}}$, $Q=\frac{x}{x^{2}+y^{2}}$

C. $P=\frac{x}{x^{2}+y^{2}}$, $Q=\frac{-y}{x^{2}+y^{2}}$

D. $P=\frac{x}{x^{2}+y^{2}}$, $Q=\frac{y}{x^{2}+y^{2}}$