Why is the sum of two rational numbers always rational?
select from the drop-down menus to correctly complete the proof.

let a/b and c/d
represent two rational numbers. this means a, b, c, and d are (integers/irrational numbers? )
and (b is not 0, d is not 0/ b and d are 0? )
the
sum of the numbers is
ad+bc/bd, where bd is not 0. because integers are closed under (addition and multiplication/ addition and division? )
the sum is the
ratio of two integers, making it a rational number.