Three different types of monthly computer passes are offered by a city's local transit authority for three different groups of passengers. How many different kinds of passes must be possible?
a. 9 possible passes
b. 18 possible passes
c. 15 possible passes
d. 6 possible passes
e. 12 possible passes
f. 11 possible passes
Additional information
This kinds of problem based on the Mathematics and it is about finding the different kinds of passes issued by the city's local transit authority.