Numbers that have the same remainder when divided by 5 - we only look at the one's digit:

0 and 5
1 and 6
2 and 7
3 and 8
4 and 9

1) Insufficient because of the numerous combinations
2) We know nothing about r and s

Together

If t = 20, then r and s can be 5 and 15 or 10 and 10
If T = 21 then r and s cannot both have a remainder of 1 and sum to 21
If T = 22 then r and s cannot both have a remainder of 2 and sum to 22
If T = 23 then r and s cannot both have a remainder of 3 and sum to 23
If T = 24 then r and s cannot both have a remainder of 4 and sum to 24

C