Let $f: R→ R$ be a function defined as $f(x)=2x^3-21x^2+36x-20,$ then :

(A) maximum value of f(x) is -3

(B) minimum value of f(x) is -128

(C) maximum value exists at x= 6

(D) minimum value exists at x= 1

Choose the correct answer from the options given below

(A), (B) Only