Which is NOT a polynomial?

A. 4/x^4 + 3/x^3− 2/x^2 −1

B. x^5 − 5x^4 + 4x^3 −3x^2

C. 3x^2 + 5x^4 + 2x − 12
D. x^3− 7x^2 + 9x −20