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.