Is the following a Unilateral or Bilateral contract?

Art and Betty are neighbors and live in an agricultural area. Art just recently had built a new red barn. In late January 2004, Betty asked Art on what terms he would build a barn on her property. Art said that because he had never built a barn for hire, he would charge Betty only $100 per square foot, about $10 more than his expected cost. Art said that he would build a barn 50’ by 50’. Betty said, "OK, if you guarantee January 1, 2005 completion.” Art agreed and asked for $10,500 in advance to purchase supplies, with any additional further payment made on completion. Betty said, "OK," and paid Art $10,500.