When a person rents a flat, he or she has to open an account with a company that provides electricity in the area. The company then bills the person based on the monthly electricity usage. The company either sends out a paper bill or can email the bill to the person. Then, the person can either pay the bill by check or can apply for a direct debit or can even pay over phone. A bill is generated at the end of each month. Use the account and entry stability pattern to draw a class diagram for this application.