Concept:The downward slope of an individual demand curve is explained by the law of diminishing marginal utility.Explanation:As a consumer increases consumption of a good, the additional satisfaction gained from each extra unit becomes smaller.Because marginal utility falls, the consumer is willing to pay less for further units.Hence, more of the good is demanded only when its price decreases.This inverse relationship between price and quantity demanded causes the demand curve to slope downward from left to right.Answer:B. Marginal utility falls as consumption increases.