Identify candidate classes whose objects participate in each scenario, and then produce the required sequence diagrams.

you have to draw a sequence diagram using an appropriate program about the case study attached below, i have already answered to question 1 and 2 you have to answer to question 3
Added on 10.12.2015 10:35
This part of the coursework is based on a familiar business domain, Cavendish Supermarket. After a short description of the environment and how transactions are processed, your tasks for the assignment is listed.
Cavendish Supermarket
The sales processing governs the sales transactions for a customer at each of the checkout registers of the supermarket. A cashier is identified to the system for as long as they are operating the register. A checkout session is the processing of all the transactions for a single customer. Each item (or some quantity of the same item at the same time) purchased by the customer is considered a single transaction.
When a bar code, UPC (Universal Product Code), or internal code number is input, a coded transaction is initiated.
When a cashier types in a unit price and a category, a price-entry transaction is initiated.
A cashier uses a scanner to read a product”s bar code and convert it to a product code number. If this operation is successful, the scanner flashes a green light. If the scanner cannot read the bar code, the scanner flashes a red light. The cashier may enter information manually through the keyboard. A UPC or internal code number may be entered manually if the scanner cannot read the bar code, or for products such as loose fruit that do not have a bar code.
If the customer is purchasing more than one of the same product, the cashier may enter the item quantity followed by the UPC or internal product code. The system must keep track of how many of the products are purchased. For example, if the price is 3 for 1.00, the charge should be 35p for the first and second items and 30p for the third. Variations are possible: a product could be priced at 40p each, Buy One Get One Free. The charge should be 40p for the first item and Op for the second. It makes no difference whether the items are scanned together or separately.
For products such as loose fruits that need to be weighed, a scale is attached to and read by the cash register. The price is determined by multiplying the measured weight by the price per unit weight.
If the UPC or internal code number is unavailable or does not exist for a product, the cashier may enter the unit price for the product, the number of items (optional), and the product category (nontaxable grocery, taxable grocery, meat, dairy, etc.).
At the end of the checkout session, the total bill is computed (including any VAT), cash payment is taken, change is computed and the cash drawer opened. A receipt is given to the customer with their change.
It is worth noting that in the overall system the inventory processing capabilities allow the store manager and the inventory manager to access the product database. The store manager sets the prices for products and the inventory manager adjusts the stock levels of items as goods are delivered.
Assuming the scope will be restricted to the checkout system; that is the system as used by cashiers:
1- Draw a use case diagram showing the checkout system.
2- Drawanactivitydiagramshowingtheend-to-endprocessofhandlingONE
customers purchases. Use swim lanes to allocate different parts of the
process to the appropriate objects.
(as these two tasks will be carried out in tutorial 3, Thursday 22nd November, they carry no marks).
3- For the following scenarios:
a. Acheckoutsessionconsistingofonesuccessfullyscanneditem
b. Acheckoutsessionconsistingofonesuccessfullyscanneditemand
one item sold by weight
identify candidate classes whose objects participate in each scenario, and then produce the required sequence diagrams.

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