The immediate short-run aggregate supply curve is horizontal because of multiple choice 1 contractual agreements for both input and output prices that span the immediate short-run timeframe and imply that prices do not change during that time. contractual agreements for both input and output prices that span the long-run timeframe and imply that prices do not change. contractual agreements for labour wages that span the immediate short-run timeframe and imply that wage costs do not change during that time. menu costs that span the immediate short-run timeframe and imply that prices do not change during that time