There are three separate sources of hazard【C1】______to the use of nuclear reaction to supply us with energy. Firstly, the radioa

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问题     There are three separate sources of hazard【C1】______to the use of nuclear reaction to supply us with energy. Firstly, the radioactive material must travel from its place of manufacture to the power station.【C2】______the power stations themselves are solidly built, the containers used for transport of the material are not. Unfortunately, there are【C3】______only two methods of transport available,【C4】______road or rail, and both of these【C5】______close contact with the general public,【C6】______the routes are【C7】______to pass near, or even through,【C8】______populated areas.
    Secondly, there is a problem of wastes. All nuclear power stations produce wastes which【C9】______will remain radioactive for thousands of years. It is【C10】______to de-active these wastes, and so they must be stored【C11】______one of the ingenious but cumbersome ways that scientists have invented. For example, they must be buried under the ground【C12】______sunk in the sea. However, these【C13】______do not solve the problem completely, they merely store it, since an earth-quake could【C14】______open the containers like nuts.
    Thirdly, there is the problem of accidental exposure【C15】______to a leak or an explosion at the power station.【C16】______with the other two hazards, this is not very likely and does not provide a serious【C17】______to the nuclear program,【C18】______it can happen, as the inhabitants of Harrisburg will tell you.
    Separately and during short periods, these three types of risk are no, great cause for concern.【C19】______, though, and especially【C20】______much longer periods, the probability of a disaster is extremely high."
【C18】

选项 A、but
B、since
C、so
D、hence

答案A

解析 前后有转折之意,应使用but。故正确答案为A。
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