$\int_0^1 \int_{y-1}^{1-y} f(x, y) d x d y=$
A. $\int_{-1}^0 \int_0^{x+1} f(x, y) d x d y+\int_0^1 \int_0^{1-x} f(x, y) d x d y$
B. $\int_0^1 d x \int_0^x f(x, y) d y$
C. $\int_{-1}^1 \int_0^1 f(x, y) d x d y$
D. $\int_0^1 \int_0^{2-x} f(x, y) d x d y$ ．