[求助]一道逻辑题。。麻烦大家给我讲讲

Critics of nuclear power complain about the allegedly serious harm that might result from continued operation of existing nuclear power plants. But such concerns do not justify closing these plants; after all, their operation has caused no more harm than that caused by pollution generated by coal and oilburning power plants, the most important other sources of energy.

Which of the following is an assumption on which the argument depends?

A. Existing nuclear power plants should be closed only if it can be conclusively demonstrated that their continued operation is likely to cause harm more serious than the harm their operation has already caused.

请问这个A选项为什么不能成为一个假设呢？ （如果把only去掉能不能成为一个假设呢）这是新东方陈向东黄本上的一道题，他那个AB的逻辑关系我怎么都搞不明白，越看越糊涂，请问我该怎么办？？谁能给我讲讲啊。。。。。