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问题 选择题

已知：镁条是银白色的固体，在空气中点燃，剧烈燃烧，发出耀眼的白光，放出大量的热，生成白色固体氧化镁，镁是制造闪光粉、轻质合金原料。判断镁条发生了化学变化的主要依据是

A．剧烈燃烧

B．耀眼的白光

C．生成白色固体

D．放出大量的热

答案

答案：C

镁带在氧气中燃烧，变化的特征是，有白色固体生成，说明了镁和氧气反应生成氧化镁，有新物质生成，是化学变化．

故选：C．

单项选择题

It is impossible to believe scientific predictions that a long "nuclear winter" would envelop the Earth as a result of nuclear war. Atmospheric scientists and weather experts cannot reliably and accurately predict tomorrow’s weather. Yet the effect of nuclear explosions on local and worldwide atmospheric conditions must follow the same laws that control everyday weather changes. If the weather cannot be predicted with present knowledge, neither can a nuclear-winter scenario.
Which of the following, if true, would most seriously weaken the argument made above that if scientists cannot reliably predict the daily weather, their predictions of a "nuclear winter" cannot be believed

A．The scientific theory of a nuclear winter uses data that is available to those who forecast the daily weather.

B．Scientists’ predictions about a nuclear winter are necessarily speculative, since they cannot be verified by harmless experimentation.

C．Weather forecasters usually do not insist that their predictions are infallible.

D．Scientific predictions of catastrophic natural events such as volcanic eruptions and earthquakes usually have less reliability than everyday weather predictions.

E．(E) The scientific theory of a nuclear winter is concerned with drastic climatic changes rather than day-to-day fluctuations in the weather.
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单项选择题

五味子的主要成分为

A．生物碱

B．有机酸

C．维生素C

D．维生素E

E．联苯环辛烯型木脂素
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