kitchen clean contracts to provide services for chef rave's restaurants at a rate of $1,000 per month to begin next month. after the first cleaning, kitchen clean complains that the conditions of the exhaust hoods were worse than either party knew. the parties agree to a new agreement for $1,500 per month during months when the hoods require cleaning and $1,000 for all other months. the parties have discharged their original contract via what method?