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:

1. A
   ${\tan }^2\theta ={\tan }^2{\theta }_1+{\tan }^2{\theta }_2$
2. B
   ${\cot }^2\theta ={\cot }^2{\theta }_1-{\cot }^2{\theta }_2$
3. C
   ${\tan }^2\theta ={\tan }^2{\theta }_1-{\tan }^2{\theta }_2$
4. D
   ${\cot }^2\theta ={\cot }^2{\theta }_1+{\cot }^2{\theta }_2$