It has been hypothesized that much of the matter in the universe is "dark",i.e,unseen.Studies have been shown that galaxies in many galaxy clusters are moving faster with respect to one other than they would if visible stars constituted all their mass.The studies suggest that the galaxies are moving under the gravitational influence of unseen mass in considerable quantity.
Which of the following is an assumption underlying the passage?
a .the aggragate mass of visible stars in the galaxies mentined can be estimated with some confidence.
b.without exception,the galaxies mentioned above move toward one another.
为什么不选b,这道题目是充分型的么？麻烦给详细解释一下推理的过程，谢谢
1。这是必要型假设，充分型假设是下面那个假设，能使原文结论合理推出
2。这道题的假设比较无聊。其他选项都较易排除，因为太荒唐。C提出原文作为证据的STUDIES可行。
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