If you think about it in a more abstract way, you will see that the following must be true:
Let a,b and c be 3 real numbers. Then the product of 2 of those must be non-negative.
Note that some products of other couple of these numbers don't necessarily have to adhere to this. The statement is that there exists one such pair among the 3. Can you think of a way to prove it? What are you options?