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【简答题】
牛场如果产后45天内左右配种,三次或三次以下输精受孕的牛比例应高于( )。
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【单选题】Which of the following statements concerning monopoly is TRUE?
A.
Monopoly firms are always larger than perfectly competitive firms.
B.
A monopoly has no rivals.
C.
Barriers to entry do not prevent other firms from entering a monopolized industry.
D.
Monopolists produce more output than a competitive market with the same demand and cost structure.