13. A monomial is defined as A. two terms. B. the difference of a constant and a variable. C. the sum of a constant and a variable. D. one term.

The correct answer is D. one term.<br /><br />A monomial is a mathematical expression that consists of only one term. It can be a constant, a variable, or a product of constants and variables. Monomials do not have any addition or subtraction operations between terms.