Excellence Middle School charges $9.00 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--9s + g
b--s + 9g
c--18(s + g)
d--9(s + g)