JEE Math Practice Question

Let S be the set of all points in (–$$\pi $$, $$\pi $$) at which the function, f(x) = min{sin x, cos x} is not differentiable. Then S is a subset of which of the following ?

1. A.$$\left\{ { - {\pi \over 2}, - {\pi \over 4},{\pi \over 4},{\pi \over 2}} \right\}$$
2. B.$$\left\{ { - {{3\pi } \over 4}, - {\pi \over 2},{\pi \over 2},{{3\pi } \over 4}} \right\}$$
3. C.$$\left\{ { - {\pi \over 4},0,{\pi \over 4}} \right\}$$
4. D.$$\left\{ { - {{3\pi } \over 4}, - {\pi \over 4},{{3\pi } \over 4},{\pi \over 4}} \right\}$$