The difference can be 1 (for the primes 2 and 3).
The difference can be 2 (for the primes 3 and 5).
The difference can be 3 (for the primes 2 and 5).
The difference can not be 7.
(if both primes are greater than 2, then they both are odd -- hence, the difference must be an EVEN number.
If one prime is 2, then the other positive integer with the difference of 7 must be 2+7 = 9, but 9 is not a prime number).
ANSWER. Option D).