Which is a counterexample to this conjecture?

The sum of any two consecutive integers is a composite number.

A.
16 + 17 = 33

B.
10 + 11 = 21

C.
6 + 7 = 13

D.

7 + 8 = 15

Question

Which is a counterexample to this conjecture? 

 

The sum of any two consecutive integers is a composite number.
	
	
 
A.
	16 + 17 = 33
	
 
B.
	10 + 11 = 21
	
 
C.
	6 + 7 = 13
	
 
D.
	

7 + 8 = 15

anonymous · Answer

Can you find any prime numbers on the right hand side?

perl · Answer

a counterexample is an example in where the conjecture is false. 
so here 6 and 7 are consecutive numbers, but their sum is prime, making the conjecture false in general . 
remember it takes only one counterexample to refute a general claim.

anonymous · Answer

It was C, I just had to read it more carefully.