9.交换积分 $\displaystyle{\int_{0}^{1} \mathrm{~d} y \int_{y}^{1} f(x, y) \mathrm{d} x}$ 的积分次序的结果为 .
A
$\displaystyle{\int_{0}^{1} \mathrm{~d} x \int_{x}^{1} f(x, y) \mathrm{d} y}$
B
$\displaystyle{\int_{y}^{1} \mathrm{~d} x \int_{0}^{1} f(x, y) \mathrm{d} y}$
C
$\displaystyle{\int_{0}^{1} \mathrm{~d} x \int_{0}^{x} f(x, y) \mathrm{d} y}$
D
$\displaystyle{\int_{0}^{1} \mathrm{~d} y \int_{0}^{1-x} f(x, y) \mathrm{d} y}$