Assume That Height Is a Function of Age and That $H=f(a)$

Multiple Choice

Assume that height is a function of age and that $H=f(a)$ is the average height (in inches) for females in the US at age $a$ years. What is the practical interpretation of $f(20)$ ?

A) The number of US females who are 20 years old.
B) The number of US females who are 20 inches tall.
C) The average height of a US female who is 20 years old.
D) The average age of a US female who is 20 inches tall.