Abnormal Behaviour Has Been Defined as Behaviour That Occurs Infrequently

Abnormal behaviour has been defined as behaviour that occurs infrequently. Which of the following examples illustrates a problem with this definition?

A) People with IQs below 70 are considered to have abnormal intellectual functioning.
B) Most people get depressed from time to time.
C) Mathematical geniuses are considered rare in the population.
D) Children often believe in the existence of monsters.
E) Anxiety disorders are relatively rare in a given population.