Monopoly is one off the peculiar (21) which can affect the sale and purchase of certain commodities. In some markets, there may be only one seller or a (22) of sellers working very closely together to control prices. The result of such monopolistic activity is to fix prices at a level (23) to the seller, a level which may bring him artificially high profits. Many governments dislike this procedure and have taken legal actions to (24) or halt such business activities. In the U. S. anti-trust laws operate to limit cartels and mergers, (25) in Britain the Monopolies Commission examines all special arrangements and mergers referred to them by the Board of Trade.This type of monopoly is not the only (26) however. There are three other forms, state, legal and natural. State monopolies are quite common nowadays, where the (27) in a particular country control industries like steel and transport or important and prestigious revises like national airlines. Legal monopolies are rather different, because the law permits certain individuals to (28) solely from their special inventions, discoveries or processes. No other person may infringe their rights in respect to (29) monopolies.Finally, natural monopoly (30) where a nation or individual possesses most of a particular mineral for reasons of geography and geology. 24（）

A. promote
B. restrict
C. impose
D. refrain