On Friday 10 December the following notice is placed in the window of Ann's art gallery: '2 copies of a very rare Blake print – £15,000 each'. Chas and Dave are very keen to acquire the prints but think that the price is too high. They each offer Ann £12,000 for a copy but she refuses to sell the prints at that price, although she says she will accept £13,500. She also says she will keep her offer to them open until 12 o'clock on the following Monday, 13 December, if they each pay her £100. Chas and Dave agree and each hands over £100. On the Saturday before the deadline, Chas and Dave have to leave the country on business but before they do so, each post a letter stating that they agree to buy one of the prints at the agreed price of £13,500. Chas' letter arrives at 9.30 on the Monday morning but Dave's letter is delayed and arrives on the morning of Tuesday 14 December.
【论述题】
Identify whether the notice Ann placed in her gallery window is an offer or an invitation to treat