The correct answer is:
Yes, it must.
Step-by-step explanation:
Rational numbers are closed under subtraction. This means that if you subtract two rational numbers, your answer will also be a rational number.
The only way you could have two rational numbers and end with a result that is not a rational number is to take the square root of at least one of them. Depending on the digits in the numbers, this would result in an irrational number.