For which functions would the appropriate domain be all positive rational numbers?

Responses
A The function f(n) models the size of a jar that holds n marbles.The function f(n) models the size of a jar that holds n marbles.
B The function f(y) models the number of inches a tree grows in n years.The function f(y) models the number of inches a tree grows in n years.
C The function f(r) models the circumference of a balloon based on radius, r.The function f(r) models the circumference of a balloon based on radius, r.
D The function f(s) models the population of mice based on the number of n snakes in an ecosystem.The function f(s) models the population of mice based on the number of n snakes in an ecosystem.
E The function f(n) models the amount of labor-hours it takes to construct n televisions that are ready to be sold.

C The function f(r) models the circumference of a balloon based on radius, r.