(Case Analysis) When the Prestons took out a variable mortgage with the First Bank of Marietta their interest rate was 9 percent. The agreement allowed First Bank to raise or lower the interest rate at any time, provided that the Prestons received 30 days advance notice. When the bank raised the interest rate to 11 percent, the Prestons refused to pay arguing that the agreement was unenforceable, because it set no limit on what interest rate they might be forced to pay. Were the Prestons correct? Explain