Answer:
D They must have common denominators.
Step-by-step explanation:
Rational expressions are fractions formed by two polynomials, one in the numerator and other in the denominator.
The numerator is the polynomial that is at the top of the fraction and denominator is the polynomial that is at the bottom of the fraction.
To add any fractions it is necessary that they have the same denominator, this applies to rational expressions, to add them it is required that they have the same polynomial in the denominator.
