Cathy (seller) offered to sell certain land to Mark (buyer) for $100,000 by signed email. The email indicated that the entire amount was due in cash at closing, and Mark responded by signed email that he accepted the offer, but payment would be half at closing, and half thirty days later. It is now five months later, Cathy never responded and Mark wants to enforce the contract.

What law applies to this sales transaction? Does Cathy have a defense or defenses to performing any contract?