Which algebraic expression is a polynomial with a degree of 5?
a. 3x^5 + 8x^4y^2 – 9x^3y^3 – 6y^5
b. 2xy^4 + 4x^2y^3 – 6x^3y^2 – 7x^4
c. 8y^6 + y^5 – 5xy^3 + 7x^2y^2 – x^3y – 6x^4
d.–6xy^5 + 5x^2y^3 – x^3y^2 + 2x^2y^3 – 3xy^5