Explanation

8) Choices A and B are correct. The question asks what the binary planet hypothesis holds. Choice A is correct: According to the first sentence of the passage, the binary planet hypothesis holds that “Earth and the Moon formed simultaneously,” that is, at
the same time. Choice B is correct: According to the first sentence of the passage, the binary planet hypothesis holds that Earth and the Moon formed “by the accretion of smaller objects,” that is, by smaller objects joining together. Choice C is incorrect: The passage does not mention the absolute sizes of Earth’s core and the Moon’s core; it only compares their sizes relative to the volumes of the two objects.

9) The questions in Choices A, B, D, and E are all answered by the giant-impact hypothesis: for Choice A, the rock that surrounded the impactor’s core “would become the Moon”; for Choice B, the impactor’s core “would fall as molten rock into the liquefied iron core of the Earth”; for Choice D, the Moon’s iron core is small relative to the Earth’s core because the matter that formed the Moon was “almost devoid of iron”; and for Choice E, the passage states that the impactor was “a large object about the size of Mars.” But nothing in the passage refers to the origin of the impactor, so Choice C is the correct answer.
Note: ETS mistakenly in the OG signed as Official Answer B, but indeed is C.

10) The passage begins by presenting the binary planet hypothesis about the formation of Earth and the Moon and claiming that the hypothesis fails to explain the disparity in the sizes of Earth’s iron core and the Moon’s iron core relative to their volumes. The passage then introduces an alternative—the giant-impact hypothesis—and argues that this alternative explains the disparity better. Thus, Choice E is correct. The second theory is not presented as having been developed out of the first, so Choice A is incorrect; only the first theory is revealed as inadequate, so Choice B is incorrect; and the two theories are not compared extensively, so Choice D is incorrect. Although “a theory [the binary planet hypothesis] is presented, and then evidence that undermines that theory [the disparity related to iron cores] is discussed,” that description fails to capture the organization of the passage as a whole, so Choice C is incorrect.