The number of passengers on a certain bus at any given time is given by the equation P = –2(S – 4)2 + 32, where P is the number of passengers and S is the number of stops the bus has made since beginning its route. If the bus begins its route with no passengers, what is the value of S when the bus has its greatest number of passengers?

a. 9

b. 6

c. 4

d.2

e.1

oa is C
I kinda solved my way through it, but can some one explain me the logic?