Duffer then rejected Slicker's bike on the ground that he had already bought Koolcat's"In Duffer's letter of October 8, what was the legal effect of the language: "This offer is not subject to countermand"?
Such language did not affect the offerer's power of revocation of the offer
Under the Uniform Commercial Code the offer was irrevocable until noon, November 12. 
Such language prevented an effective acceptance by Slicker prior to noon, November 12. 
At common law, such language created a binding option in Slicker's favor. 