Help! Will mark brainliest

A theater company charges $7.50 per ticket for people of all age groups. If c represents the total number of children who bought tickets and a represents the total number of adults who bought tickets, which expression best represents the total money earned by the company from the sale of tickets?

A: 7.5c + a
B: c + 7.5a
C: 15(c + a)
D: 7.5(c + a)