The passage says- according to some skeptics the reason for the whirling of gas is likely to be a cluster of stars. So the assumption is that the cluster of stars causes the whirling of gas. To weaken this hypothesis, we've to find a reason that negates/undermines the skeptics' hypothesis. Choice B ...

Readin Comprehension- please help me!

Theorists are divided about the cause of the Permian mass extinction. Some hypothesize that the impact of a massive asteroid caused the sudden eradication of most species. However, a look at the carbon-isotopic record suggests that existing plants communities were decimated and revived several times...