JEE Main 2019 Mathematics question

Let f(x) = 15 − |x − 10|; x ∈ R. Then the set of all values of x, at which the function, g(x) = f(f(x)) is not differentiable, is:

• (A){10, 15}
• (B){5, 10, 15}Correct
• (C){10}
• (D){5, 10, 15, 20}