In economics, demand implies something slightly different from the common meaning of the t

erm. The layman, for example, often used the term to mean the amount that is demanded of an item. Thus,if the price were to decrease and individuals wanted more of the item, it is commonly said that demand increases. To an economist, demand is a relationship between a series of prices and a series of corresponding quantities that are demanded at these prices. If one reads the previous sentences carefully, it should become apparent that there is a distinction between the quantity demanded and demand. This distinction is often a point of confusion and we all should be aware of and understand the difference between these two items. We repeat, therefore, that demand is a relationship between price and quantities demanded,and therefore suggests the effect of one(e.g., price)on the other(e.g. quantity demanded). Therefore, knowledge of the demand for a product enables one to predict how much more of a product will be purchased if price decreases. But the increase in quantity demanded does not mean demand has increased, since the relationship between price and quantity(i.e., the demand for the product)has not changed. Demand shifts when there is a change in income, expectations, tastes,etc., such that a different quantity of the product is demanded at the same price. In almost all cases, a consumer wants more of an item if the price decreases. This relationship between price and quantity demanded is so strong that it is referred to as thelaw of demand". This"law"can be explained by the income and substitution effects. The income effect occurs because price increas-es reduce the purchasing power of the individual and,thus,the quantity demanded of goods must de-crease. The substitution effect reflects the consumers desire to get the "best buy". Accordingly, if the price of product A increases, the individual will tend to substitute another product and purchase less of goods A. The negative correlation between price and quantity demanded is also explained by the law of diminishing marginal utility. According to this law, the additional utility the consumer gains from consum-ing a product decreases as successively more units of the product are consumed. Because the additional units yield less utility or satisfaction, the consumer is willing to purchase more only if the price of the product decreases. Economists distinguish between individual and market demand. As the term implies, individual demand concerns the individual consumer and illustrate the quantities that individuals demand at different prices. Market demand includes the demand of all individuals for a particular goods and is found by summing the quantities demanded by all individuals at the various prices. The other side of the price system is supply. As in the case of demand, supply is a relationship between a series of prices and the associated quantities supplied. It is assumed that as price increases the individual or firm will supply greater quantities of a product. There is a positive correlation between quantity supplied and product price. Economists also distinguish between a change in supply and quantity supplied. The distinction is similar to the one made with respect to demand. Also, as in the case of demand, economists distinguish between individual firm supply and market supply, which is the summation of individual supply. Taken together, supply and demand yield equilibrium of price and quantity. Equilibrium is a state of stability, with balanced forces in which prices and quantity will remain constant. Moreover, there are forces in the market that will act to establish equilibrium if changes in demand or supply create disequilibrium. For example, if prices are above equilibrium, the quantity supplied exceeds quantity demanded and surpluses occur that have a downward pressure on prices. These pressures will persist until equilibrium is established. If prices are below equilibrium, the product will become scare and there will be an upward pressure on price.

According to the passage, demand_____.

A．increases as price decreases

B．can be predicted if there is a change in price

C．remains constant at the same price

D．is influenced by changes in the consumer"s income and tastes