题目内容：
根据材料，回答问题。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 competes 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.
请在第____处填上正确答案。 A．to
B．at
C．of
D．on
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