设 $P=P(x, y, z), Q=Q(x, y, z)$ 均为连续函数， $\Sigma$ 为曲面
$$
z=\sqrt{1-x^2-y^2}(x \leq 0, y \geq 0)
$$
的上侧，则 $\iint_{\Sigma} P \mathrm{~d} y \mathrm{~d} z+Q \mathrm{~d} z \mathrm{~d} x=$
$\text{A.}$ $\iint_{\Sigma}\left(\frac{x}{z} P+\frac{y}{z} Q\right) \mathrm{d} x \mathrm{~d} y$
$\text{B.}$ $\iint_{\Sigma}\left(-\frac{x}{z} P+\frac{y}{z} Q\right) \mathrm{d} x \mathrm{~d} y$
$\text{C.}$ $\iint_{\Sigma}\left(\frac{x}{z} P-\frac{y}{z} Q\right) \mathrm{d} x \mathrm{~d} y$
$\text{D.}$ $\iint_{\Sigma}\left(-\frac{x}{z} P-\frac{y}{z} Q\right) \mathrm{d} x \mathrm{~d} y$