13. Ticket sales at the First Run Theater total at least $7,600 per week. An adult's ticket costs $7.50 and a child's ticket costs $4.00. If a represents the number of adult tickets sold in a week and c represents the number of child tickets, which algebraic sentence represents the money received each week from ticket sales?
a) 7.50 a + 4.00 c= 7,600
b) 7.50 a + 4.00 c > 7,600
c) 7.50 a + 4.50 c > 7,800
d) 7.50 a + 4.00 c < 7,600