On 4 April, Alison Offers Betty an Antique Clock for £2000

On 4 April, Alison offers Betty an antique clock for £2000. Betty spends a week mulling over the offer. On 13 April, she sends Alison a letter stating she'd be willing to buy it for £1500. The next day, while looking through an antiques catalogue she sees that a similar clock has recently been auctioned off at £5000. Betty realizes that if Allison refuses her counter offer she may lose the clock altogether. She immediately sends another letter on 14 April accepting the original offer. Betty's first letter containing the counter offer arrives on 16 April; her second letter accepting the original offer arrives on 18 April. Have the parties concluded a contract and, if so, when?

A) No, Betty's counter-offer killed off Alison's original offer on 13 April.
B) Yes, Betty has accepted Alison's original offer of £2000 on 14 April.
C) No, Betty's counter-offer killed off Alison's original offer on 16 April.
D) Yes, Betty accepted Alison's original offer of £2000 on 18 April.