若函数z=(x-y)/(x+y)，则dz=（）A. (2(xdx-ydy))/((x+y)^2)B. (2(ydy-xdx))/((x+y)^2)C. (2(ydx-xdy))/((x+y)^2)D. (2(xdy-ydx))/((x+y)^2)

若函数$z=\frac{x-y}{x+y}$，则$dz=$（）

A. $\frac{2(xdx-ydy)}{(x+y)^2}$

B. $\frac{2(ydy-xdx)}{(x+y)^2}$

C. $\frac{2(ydx-xdy)}{(x+y)^2}$

D. $\frac{2(xdy-ydx)}{(x+y)^2}$