Which of the following statements is true?
The rational number system is closed under addition, because the sum of two rational numbers is always an irrational number.
The rational number system is closed under addition, because the sum of two rational numbers is always a rational number.
The rational number system is not closed under addition, because the sum of two rational numbers is not always an irrational number.
The rational number system is not closed under addition, because the sum of two rational numbers is not always a rational number.
