第6部分：完形填空（第51～65题，每题1分，共15分）
阅读下面的短文，文中有15处空白，每处空白给出了4个选项，请根据短文的内容从4个选项中选择1个最佳答案，涂在答题卡相应的位置上。
Cost as a Factor in Supply
In a purely competitive market, the supplier of goods and services has no control over the market price, because he produces too little to influence market conditions. With no difference between his products and the products（51） his competitors, he will sell nothing if he charges above the market price and he will sell all if he charges at or（52） the market price. However, in considering the price, he must take the（53） of production into consideration. There are times when he may be willing to sell below his cost. This might happen when prices tumble for（54） he believes will be a short time. However, no business person can（55） to lose money for a prolonged period. He must be constantly（56） of his costs in relation to the market price if he is to compete successfully and earn a profit.
Many people have the impression that as production increases, costs per unit decrease. （57） mass production has made this true in certain industries and at certain levels of production,（58） logic and practical experiences have shown that costs per unit begin to rise beyond a certain level of production. Some economists（59） to this principle as the law of increasing costs.
The reason costs rise as production goes up is（60）. However, it is easy to recognize that as production goes up, the need for additional factors of production will also grow, （61） competitive bidding in the marketplace for the factors of production. If a producer needs（62） skilled labor to produce more, and none of this labor is unemployed, the producer will have to get（63） from other sources. This can be done by（64） higher wages. Higher bidding would also apply to the other factors of production. We must also recognize that not all labor is equally productive,（65） not all land is equally fertile and not all ore （矿石） is equally rich in the mineral wanted.
51 A to B at C of D on
52 A below B beneath C over D above
53 A price B cost C worth D profit
54 A that B why C what D if
55 A afford B pretend C offer D decide
56 A sure B afraid C aware D suspicious
57 A Because B since C When D While
58 A both B neither C none D any
59 A resort B refer C turn D attend
60 A clear B simple C difficult D complex
61 A bringing B resulting in C including D carrying out
62 A less B numerous C more D many
63 A them B these C it D those
64 A offering B cutting C reducing D having
65 A as if B just as C because D while
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