A **prime number** is a number greater than 1 which has only 1 and itself as factors.

A

**composite number** is a number which has factors other than 1 and itself.

In addition to 1 and 9, 9 has the factor pair 3 and 3.

So, 9 is a composite number.

Aside from 1 and 17, there are no other factor pairs for 17.

So, 17 is a prime number.