单项选择题

Solid fuel rockets are expensive to operate because of their ______.

A．size
B．fuel
C．construction
D．complicated engines

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During the 20th century Europe has experienced periods of considerable economic growth and prosperity, and industrial development has proliferated much more widely throughout the continent: but continued economic development in Europe has been handicapped to a large degree by its multi- national character--which has spawned economic rivalries among states and two devastating World wars--as well as by the exhaustion of many of its resources and by increased economic, competition from overseas. Governmental protectionism, which has tended to restrict the potential market for a product to a single country, has deprived many industrial concerns of the efficiencies of large-scale production serving a mass market ( such as is found in the United States). In addition, enterprise efficiency has suffered from government support and from a lack of competition within a national market area. Within individual countries there have been growing tensions between regions that have prospered and those that have not. This core-periphery problem has been particularly acute in situations where the contrasting regions are inhabited by different ethnic groups.

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