In a system for a restaurant, the main scenario for placing an order is given below:

(a) Customer reads menu
(b) Customer places order
(c) Order is sent to the 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 at least how many objects among whom the messages will be exchanged.