Lin offers to sell his prized collection of stamps to Dora and Brad for $1000. He sent each of them the offer on April 10 in a mailed letter that said "I will sell it to the first person who agrees to my terms." Dora agreed to the offer in a mailed letter that was sent on April 13. Brad met up with Lin for coffee on April 14 and agreed to the offer then. Lin received Dora's letter on April 15. With whom and when was the contract formed?

a) Brad on April 10

b) Dora on April 15

c) Brad on April 14

d) Dora on April 13