A prime number is not divided by another number except 1 and itself.
33 is not prime as it is divisible by 3 and 11.
45 is not prime as it is divisible by 3, 5, 9.
72 is not prime as it is divisible by 2, 3, 4, 6, 8, 9, 18, 36.
97 is a prime as it is not divisible by any other number except 1 and itself.