It is common for supermarkets to carry both generic (store-label) and brand-name (producer-label) varieties of sugar and other products. many consumers view these products as perfect substitutes, meaning that consumers are always willing to substitute a constant proportion of the store brand for the producer brand. consider a consumer who is always willing to substitute four pounds of a generic store-brand sugar for two pounds of a brand-name sugar. do these preferences exhibit a diminishing marginal rate of substitution between store-brand and producer-brand sugar?