Rocky Mountain Races, Inc., sponsors the "Pioneer Trail Ultramarathon" with an advertised first prize of $10,000.The rules require the competitors to run 100 miles from the floor of Blackwater Canyon to the top of Pinnacle Mountain.The rules also provide that Rocky reserves the right to change the terms of the race at any time.Monica enters the race and is declared the winner.Rocky Mountain offers here a prize of $1,000 instead of $10,000.Did Rocky and Monica have a contract?