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【单选题】
可控整流电路和直流斩波电路，都是把电能的形式变换为直流电能。但是，二者主要特点的不同之处在于（）。A.
采用的变压器不同，前者采用工频变压器，后者采用高频变压器，后者体积更小
B.
采用的滤波器体积不同，后者滤波元件的尺寸更小
C.
采用的开关器件的类型不同，后者采用器件，可控性更好
D.
采用的工作频率不同，后者更高，电压动态响应更好
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【单选题】The book, ____ are true stories about great scientists of the 20 th century, is well worth reading.
A.
which
B.
what
C.
in which
D.
of which
【简答题】Underwater cameras are helping scientists make ______, but diving expeditions are still the most useful way to gather information.
【单选题】______ the beginning of the 19th century did scientists know that all matter is made up of atoms. [ ]
A.
At
B.
By
C.
Up to
D.
Not until
【单选题】On the basis of their models, scientists are of the opinion that______.
A.
the climate of the world should be becoming cooler
B.
it will take thousands of years for the inertia of the earth's climate to take effect
C.
the man-made warming effect helps to increase the solar effect
D.
the new Ice Age will be delayed by the greenhouse effect
【简答题】_____the beginning of the 19 th century did scientists know that all matter is made up of atoms. A,At B,By C,Up to D,Not until
【单选题】With the population explosion, scientists will have to ______ new methods of increasing the world's food supply.
A.
come up with
B.
carry out
C.
lead to
D.
stick to
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