Concept: The base 10 system is common because it is linked to human counting habits.Explanation:Early humans naturally counted using their fingers.Since most people have exactly 10 fingers, counting in groups of 10 became familiar.This made base 10 the most widely adopted number system.Therefore, its uniqueness comes from human anatomy, not from any special mathematical property of the number 10.Answer: A. There are only 10 fingers to every human.