This question takes a bit of reasoning, but once you establish a
pattern, the problem opens up for you. Start by ruling out any even
number because adding two even numbers will create a number divisible by
2. Then add the smallest odd prime number, 3, to each of the numbers:
3 + 2 = 5
3 + 4 = 7
So 3 works, but when you try 5, 7, 11, and 13 you quickly see that
there are no more reasonable assumption possible.
(B)