Choose the simplified form of the expression.

5x^4 + 6x^2 - 2x + x^3

A.. None of these
B. -2x^4 + 7x^2 + 6x
C. The expression is simplified.
D. 5x^4 + 7x^2 - 2x
E. x^4 + 7x^2 +6x