What is the sum of the polynomials? (9x4−13x3−x−7)+(7x3−2x+1) A.) −6x^6+9x^4−3x^2−6 B.)−6x^6+9x^4−x−8 C.)9x^4−6x^3−x−8 D.)9x^4−6x^3−3x−6