GM sends a purchase order to its supplier, Tonka, to buy 2 million tire parts. The purchase order says that payment will be made in 30 days. Tonka responds with a sales invoice that says payment instead is due in 15 days. Which is true:

• The parties have a contract, and payment is due in 15 days unless GM complains about it
• There is no enforceable contract between the parties
• The parties have a contract, and payment will be due at whatever time is considered reasonable under UCC gap-filling rules, because the inconsistent payment terms will be "knocked out."