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.