Maria has decided to sell her bicycle. She pins up posters around Ryerson that advertise the bicycle. The posters say: "Breaking up, trading up to a Vespa, yay! Selling my awesome bicycle. Asking $500." The posters also have Maria's contact information on them. Parmpreet sees the poster and she texts Maria and says, "I would like to buy your bicycle for $500! Meet me in front of my residence at Ryerson on Tuesday at 6:15 pm with the bicycle."

Maria receives Parmpreet's text and replies "Okay, you seem nice. The bicycle is yours, but since I forgot about the tax on my Vespa the price is actually $525 for the bicycle". Under the circumstances, which of the following is TRUE?

a. A valid acceptance has occurred.

b. A valid acceptance has not occurred.

c. The parties have stopped negotiating.

What should be the answer of this question and the legal rules and facts to this case.