In a system for a restaurant, the main scenario for placing order is given below:
- Customer reads menu
- Customer places order
- Order is sent to kitchen for preparation
- Ordered items are served
- Customer requests for a bill for the order
- Bill is prepared for this order
- Customer is given the bill
- Customer pays the bill
A sequence diagram for the scenario will have at least how many objects among whom the messages will be exchanged
- $3$
- $4$
- $5$
- $6$