SAT Question 12030076: Central Ideas And Details answer and explanation

Choice D is the best answer because it most accurately states the main idea of the text. The text begins by mentioning NASA’s Aspera mission, which will investigate the low-density gas that makes up the circumgalactic medium (CGM). According to the text, this mission will focus on a portion of the CGM’s gas that exists in a “warm-hot” phase; this “warm-hot” gas has not been previously observed, but it is thought to make up most of the mass of galaxies and play a part in star formation. Finally, the text mentions a telescope capable of examining this previously unobservable “warm-hot” gas: the Aspera mission will use this telescope in the hope of answering questions about galaxy formation and change. Therefore, the main idea of the text is that the Aspera mission is likely to produce the first direct observations of CGM gas in the “warm-hot” phase, which likely has an important role in the evolution of galaxies.