Data Sufficiency (DS) | Re: If r and s are integers, is r+s divisible by 3?

The concept in this question is basically testing three fundamental scenarios

Scenario 1

X + Y / some multiple K- in order for the sum of X and Y to be a multiple ofK both X and Y must be a multiple of K

Scenario 2

X + Y/ some multiple K- if either X or Y is not a multiple of K and the other is ...then the sum of X and Y cannot be a multiple of k

Scenario 2

X +Y/ some multiple K - Actually,
...

---