[单项选择]It may be necessary to stop ______ in the learning process and go back to the difficult points in the lessons.
A. at a distance
B. at intervals
C. at case
D. at length

[单项选择]Although learning abilities may differ and talents may vary, at no time during the lifetime of a human being does learning ever stop.（）
A. 学习困难，内容很多，但在生活中没有时间也必需坚持学习。
B. 学习能力不同，方法各异，但人们一生总无论如何都不能终止学习。
C. 尽管人们学习能力不同，天赋各异，但学习会伴随人们的一生。
D. 学习能力不强，方法不同，但一个人一生中都要坚持学习。
E. 学习困难，内容很多，但没有时间学习，生命就完全失去意义。

[单项选择]

It may be necessary, in order that future (51) can enjoy clean air, that more of the (52) is borne by long-term central government fund. As air pollution varies enormously from place to place and can, indeed, be very local, it may also be necessary for the central government to (53) money so that local authorities can (54) special local problems. That these can be significant was shown by the report made in 1970 (55) behalf of the London Boroughs Association. This (56) that in central London since 1958 smoke concentrations have decreased by 80 percent, whereas sunshine has increased by 70 percent and winter visibility has (57) threefold. There have been (58) in mortality and hospital admissions (59) with air pollution and in the response to it of bronchitis, as well as clear if less documented increases in plant types and bird (60) in the cities.

51（　）.
A. habitats 
B. inhabitants 
C. inhabitant 
D. inhabit

[单项选择]