设二维随机变量(X, Y)的分布函数为F(x, y) = a ( b + arctan (x)/(2) ) ( c + arctan (y)/(3) ), -infty A. (1)/(pi)B. piC. (1)/(pi^2)D. 0

设二维随机变量$(X, Y)$的分布函数为$F(x, y) = a \left( b + \arctan \frac{x}{2} \right) \left( c + \arctan \frac{y}{3} \right), -\infty < x, y < +\infty$, 则$a = ()$

A. $\frac{1}{\pi}$

B. $\pi$

C. $\frac{1}{\pi^2}$

D. $0$