Solve the Problem $\theta$ Is $f ( \theta ) = \cot \theta$

Multiple Choice

Solve the problem.
-For what numbers $\theta$ is $f ( \theta ) = \cot \theta$ not defined?

A) integral multiples of $\pi \left( 180 ^ { \circ } \right)$
B) all real numbers
C) odd multiples of $\pi \left( 180 ^ { \circ } \right)$
D) odd multiples of $\frac { \pi } { 2 } \left( 90 ^ { \circ } \right)$