Answer:
The number 1 is common factor between any set of numbers, but, sometimes is the only common factor.
Explanation:
This happens when the numbers do not have common prime factors, like 5 and 7.
Since 1 is a prime number, you can consider it the common primer factor of every set of numbers.
Remember:
If 'a' is a number, a prime factor of it would be 1, since a*1=a