设z=(x+y)/(x-y)，则dz等于()

设z=(x+y)/(x-y)，则dz等于()
A、2/(x-y)²(xdx-ydy)
B、2/(x-y)²(xdy+ydx)
C、-2/(x-y)²(xdx-ydy)
D、2/(x-y)²(xdy-ydx
【正确答案】：D