设δ=√(1-x2-y2),则dδ=()

设δ=√(1-x2-y2),则dδ=()
A、-x/√(1-x2-y2)dx
B、-y/√(1-x2-y2)dy
C、(dx+dy)/√(1-x2-y2)
D、-(xdx+ydy)/√(1-x2-y2)
【正确答案】:D