Select a counter-example that makes the conclusion false.. 3 is prime, 5 is prime, 7 is prime. conclusion: all odd numbers are prime.. counterexample: .

We must select a counter-example that makes the conclusion false. 

3 is a prime, 5 is a prime, and 7 is a prime. 
The conclusion is that all odd numbers are prime numbers. 
A counter example is that 9 is an odd number but it is NOT a prime number.