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 sent 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.