Explanation
Correct answer: C.
Choice C is the best answer. The sentence specifies the reason the Pleiades’ appearance changed, noting that two of the cluster’s stars have moved so close together that they now appear as one star.
Why the other choices are wrong
Choice A
Choice A is incorrect. The sentence specifies how ancient Native American and Australian Aboriginal cultures described the Pleiades; it doesn’t specify the reason the Pleiades’ appearance changed.
Choice B
Choice B is incorrect. The sentence describes the appearance of the Pleiades today; it doesn’t specify the reason the Pleiades’ appearance changed.
Choice D
Choice D is incorrect. The sentence explains why two of the Pleiades’ stars appear to be a single star; it doesn’t specify the reason the Pleiades’ appearance changed.