The paragraph is about Martian brines. Of the sentences given, B is the one that explains what these are: Mars has abundant reserves of subsurface water ice and some amount of liquid water in the form of brines . So, B is the first sentence of the paragraph. Now, while sentences C and D relate to the salt content of these brines, A and C relate to their oxygen content. Consider sentence C. This starts off referring to "these pools of salty liquid". Sentence D explains why the Martian brines remain liquid on the planet's frigid surface. D explains the reason the brines are in liquid form; C carries forward this idea and goes on to explain how the brines absorb oxygen from Mars' atmosphere. So, C follows D. Between A and C, we see that A adds to the idea in C about the oxygen content of the brines. So, A follows C. BDCA is hence the correct order. Correct Answer: BDCA
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