Which of the following equations demonstrate that the set of polynomials is not closed under the certain operations?
a. division: (x^2+2x) / (x+1) = x+ x/x+1
b. multip.: (3x^4++x^3)= -6x^6+ x^7+x^6
c. addition: (3x^4+x^3)+(-2x^4+x^3)= x^4 + 2x^3
d. multip.: (x^2+2x)(x+1)= x^3 +3x^2+2x