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交换积分次序，则二次积分 $\int_0^1 d y \int_y^{\sqrt{y}} f(x, y) d x=(\quad)$ ．
A. $\int_0^1 d x \int_{\sqrt{x}}^x f(x, y) d y$     B. $\int_0^1 d x \int_x^{\sqrt{x}} f(x, y) d y$     C. $\int_0^1 d x \int_{x^2}^x f(x, y) d y$     D. $\int_0^1 d x \int_x^{x^2} f(x, y) d y$         
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