If `theta_1` and `theta_2` be the apparent angles of dip observed in two vertical planes at right angles to each other, then the true angle of dip `theta` is given by
A. `cot^(2)theta=cot^(2)theta_(1)+cot^(2)theta_(2)`
B. `tan^(2)theta=tan^(2)theta_(1)+tan^(2)theta_(2)`
C. `cot^(2)theta=cot^(2)theta_(1)-cot^(2)theta_(2)`
D. `tan^(2)theta=tan^(2)theta_(1)-tan^(2)theta_(2)`
