Craig uses a ruler to determine the length of two pieces of metal. He records the length of each piece of metal as a rational number. Which statement best explains whether the sum of the two lengths Craig recorded must also be a rational number?

A. When adding two rational numbers a/b and c/d, the numerators a and c do not have to be integers. Therefore, the sum does not have to be a rational number.

B.When adding two rational numbers a/b and c/d, the common denominator bd does not have to be an interger. Therefore, the sum does not have to be a rational number.

C. When adding two rational numbers a/b and c/d, the sum is ac/bd, and both the numerator and denominator are integers. Therefore, the sum must be a rational number.

D. When adding two rational numbers a/b and c/d, the sum is ad+bc/bd, and both the numerator and denominator are integers. Therefore, the sum must be a rational number.