The set s consists of all odd multiples of 3: s = { -9, -3, 3, }. if the integers x and y are in s, then which of the following must also be in s?

a) xy
b) x + y
c) x - y
d) x/y
e) -x - y

explain too, .

I think it is gonna be xy

because if ur set contains all odd multiples of 3 (-9,-3,3,9,15,21,27...
and u pick any 2 of those numbers and multiply them u will end up with an odd numbered multiple of 3

and if u pick out any 2 numbers in ur set..and do any of the other things in ur answer choices, it will not always be an odd numbered multiple of 3