If $\tan^{-1}x+\tan^{-1}y=\frac{4\pi}{5},then\;\cot^{-1}x+\cot^{-1}y$ equals:

Question

$$(A)\quad\frac{\pi}{5}\quad(B)\quad\frac{2\pi}{5}\quad(C)\quad\frac{3}{5}\quad(D)\quad{\pi}$$