The prime factorizations of 54 and 72 are shown below.
 Prime factorization of 54: 2, 3, 3, 3
 Prime factorization of 72: 2, 2, 2, 3, 3
 Using the prime factorizations, what is the greatest common factor of 54 and 72?
 O 2'3
 O 2'3'3
 O 2'2'3'3
 O 2'2'2'3'3'3