In a system for a restaurant, the main scenario for placing order is given below : ;
(a)    Customer reads menu
(b)    Customer places order
(c)    Order is send to kitchen for preparation
(d)    Ordered items are served
(e)    Customer requests for a bill for the order 
(f)    Bill is prepared for this order 
(g)    Customer is given the bill
(h)    Customer pays the bill
A sequence diagram for the scenario will have atleast how many objects among whom the messages will be exchanged.