Let $f ( x ) = \ln ( \cosh x )$

Multiple Choice

Let $f ( x ) = \ln ( \cosh x )$ Then $f ^ { \prime } ( x )$ is

A) $\operatorname { csch } x$
B) $\sinh x$
C) $\operatorname { sech } x$
D) $\tanh x$
E) $\operatorname { coth } x$