Let S be the set of all points in (-π,π) at which the function, f(x) = min{sin x, cos x} is not differentiable. Then S is a subset of which of the following?
a. {-3π/4, -π/4, π/4, 3π/4}
b. {-3π/4, -π/2, π/2, 3π/4}
c. {-π/2, -π/4, π/4, π/2}
d. {-π/4, 0, π/4} ​