Suppose you wish to model the elevator system of a multi floor building. The system is composed of ra floors and n elevators. Each elevator has a set of buttons, one for each floor. The buttons light up when pressed and cause the elevator to visit the corresponding floor. The light goes out upon the elevator's reaching the floor, Each floor (except for the ground and top floors) has two buttons, one to request an up elevator and one to request a down elevator. The illumination goes out when the elevator visits the floor and either is moving in the desired direction or has no outstanding requests. Each elevator has an emergency button that, when pressed, causes a warning signal to be sent to the site man-ager. The elevator is then deemed “out of service.” Each elevator has a mechanism to cancel its out-of-service status. ' a. Model this system in an object-oriented style. b. Suppose that the elevators are divided into two sets, the first comprising the elevators that serve floors zero through the second comprising those which serve floors ra: through ra. What would change in your design to accommodate this feature?