An online data base charges $25 an hour during the day and $12.50 an hour at night. if a research company paid $250 for 12 hours of use, which system of equations could be used to determine the number of hours charged at the day rate (d) and at the night rate (n)?

a)
d + n = 12
25d + n = 250
b)
d + n = 12
25d + 12.50n = 250
c)
d − n = 12
25d + 12.50n = 250
d)
d + n = 250
25d − 12.50n = 250