已知 $f(t)$ 连续, 令 $F(x,y)=\int_{0}^{x-y}(x-y-t)f(t)\,dt$, 则( )
- A. $\frac{\partial F}{\partial x}=\frac{\partial F}{\partial y},\frac{\partial^{2}F}{\partial x^{2}}=\frac{\partial^{2}F}{\partial y^{2}}.$
- B. $\frac{\partial F}{\partial x}=\frac{\partial F}{\partial y},\frac{\partial^{2}F}{\partial x^{2}}=-\frac{\partial^{2}F}{\partial y^{2}}.$
- C. $\frac{\partial F}{\partial x}=-\frac{\partial F}{\partial y},\frac{\partial^{2}F}{\partial x^{2}}=\frac{\partial^{2}F}{\partial y^{2}}.$
- D. $\frac{\partial F}{\partial x}=-\frac{\partial F}{\partial y},\frac{\partial^{2}F}{\partial x^{2}}=-\frac{\partial^{2}F}{\partial y^{2}}.$