设f(x，y)是连续函数，a﹥0则∫0adx∫0xf(x,y)dy等于()

设f(x，y)是连续函数，a﹥0则∫₀^adx∫₀^xf(x,y)dy等于()
A、∫₀^ady∫₀^yf(x,y)dx
B、∫₀^ady∫_y^af(x,y)dx
C、∫₀^ady∫_a^yf(x,y)dx
D、∫₀^ady∫₀^af(x,y)dx
【正确答案】：B