Assume A.
Deduce a contradiction.
This proves A is false.
Example:
Assume there is a greatest prime number, P.
But then P! is divisible by every prime number.
So P! + 1 is divisible by no prime number (other than 1),
So P! + 1 is prime.
So by the assumption, P! + 1 is not greater than P.
But P! + 1 is greater than P.
This is a contradiction.
So the assumption is false.