Which of the polynomials below is a 3rd degree polynomial?

Select one:
a. 5x 3 − 6x 4 + 7x 2
b. 3x 2 − 5x + 7
c. 5x 2 − 6x 3 + 7x − 8
d. 3x 3 + 4x 5 + 6