If `alpha` is the angle subtended at `P(x_(1),y_(1))` by the circle `S-=x^(2)+y^(2)+2gx+2fy+c=0` then
A. `cotalpha=(sqrtS_(1))/(sqrt((g^(2)+f^(2)-c)))`
B. `cot""(alpha)/(2)=(sqrtS_(1))/(sqrt((g^(2)+f^(2)-c)))`
C. `tanalpha=(2sqrt((g^(2)+f^(2)-c)))/(sqrtS_(1))`
D. `alpha=2tan^(-1)((sqrt((g^(2)+f^(2)-c)))/(sqrtS_(1)))`
