If $ \tan \theta = \large\frac{20}{21}$ then $ \large\frac{\cos \theta - \sin \theta}{\cos \theta +\sin \theta}$

Question

$\begin{array}{1 1}(A) \; \large\frac{1}{41} \\(B)\;\large\frac{1}{20} \\(C)\;\large\frac{1}{21} \\(D)\; \large\frac{1}{20} \end{array}$