Collins contracts with Baxter for Baxter to prepare Collins's income tax returns in exchange for payment of $500. Collins's tax return is due on April 15. On April 14, Baxter calls Collins and tells her that he will need to be paid $900 to complete preparation of Collins's tax return. Collins agrees to pay Baxter $900. Choose the best answer: The contract is enforceable because the $900 price was bargained for. The contract is enforceable because Baxter relied upon Collins's promise to pay an additional $900. The contract is unenforceable because Baxter's promise is illusory. The contract is unenforceable because Baxter had a pre-existing duty to complete the tax returns.