Which of the Following Is an Example of a Contract

Which of the following is an example of a contract offer?

A) A notice on a Web site offering to sell a mobile phone for $400.
B) British Petroleum asking companies to quote for its new venture.
C) Greg puts up a notice on a tree in his locality offering a reward of $100 for anyone who finds his missing cat.
D) On January 1, a company declares a bonus of $1,000 for all its employees who complete a year in the company since the date of announcement.
E) Tixie writes an e-mail to sell her bike for $1,000, but forgets to send it. Coincidentally, her classmate's mother wants to buy a bike for her kids and sends an e-mail to Tixie saying that she wants to buy her bike for $1,000.