Which of the following equations demonstrate the set of polynomials is not closed under the certain operations?

A. Subtraction: (3x⁴ + x³) - (-2x⁴ + x³) = 5x⁴
B. Division: (x² - 5x + 3) + (x - 2) = x - 3 + -3/x - 2
C. Multiplication: (x² - 5x + 3)(x - 5) = x³ - 10x² + 28x - 15
D. Addition: (x² + x) + (x + 1) = x² + 2x + 1