the principal of a local middle school arranged for 5 buses to take 140 students on a field trip. of the 5 buses, 4 buses were completely full. only 12 students rode on the 5th bus. if b represents the maximum number of students who can ride on each bus, which equation can you use to find the value of b?

a) b+12=140

b)140-12b=4

c)4b+12=140

d)140+4b=12