Cost as a Factor in SupplyIn 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 product and the products (19) his competitors, he will sell nothing if he charges above the market price and he will sell all if he charges at or below the market price. However, in considering the price, be must take cost of production (20) . There are times when he may be willing to sell below his cost. This might happen when prices tumble for (21) a short time. However, no business person can (22) lose money for a prolonged (延长的) period. He must (23) 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 (24) production increases, costs per unit decrease. (25) mass production has made this true in certain industries and at certain levels of production, (26) logic and practical experience have shown that costs per unit begin to rise beyond a certain level of production. Some economists (27) this principle as the law of increasing costs.The reason why (28) rise as production goes up is complex. However, it is easy to recognize that as production goes up, the need for additional factors of production will also grow, resulting (29) competitive bidding (出价) in the marketplace for the factors of production. If a producer needs (30) skilled labour to produce more, and none of this labour is unemployed, the producer will have to get (31) from other sources. This can be done by (32) higher wages. Higher bidding would also apply to the other factors of production. We must also recognize that not all labour is equally productive, (33) not all land is equally fertile (肥沃的) and not all ore is equally rich in the mineral wanted. 23（）

A. constantly aware
B. constantly knowledgeable of
C. be constantly aware of
D. constantly aware of