Excellence Middle School charges $7.50 for students from all grade levels to attend the school dance. If s represents the number of seventh grade students who came to the dance and g represents the number of eighth grade students at the dance, which expression best represents the total amount of money collected for attendance at the dance? a 7.5s+g
b s+7.5g
c 15(s+g)
d 7.5(s+g)