If $p>0$ and$ q>0$, And $p^2-q^2$ is a prime number, then $p-q$ is...? Please help 
The answer is part $d)$none of the above, How do i proceed with such questions?

Note that $$p^2-q^2=(p-q)(p+q)$$
From the definition of prime numbers, it follows that $p-q=1$. 
If $p-q \neq 1$, then $p-q$ is a number that divides $p^2-q^2$ that is not $1$ or $p^2-q^2$. 
