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A. Deep below a south pole ice cap. In 2018, researchers using ice-penetrating radar on the Mars Express Orbiter found evidence for a massive liquid water lake underneath 1.5 kilometers of ice.
C. River delta. Jezero Crater is home to an ancient river delta, which deposited sediment into an ancient lake and could hold evidence of past life on Mars — if life ever evolved there.
B. Carbon dioxide. In winter, Mars becomes cold enough that carbon dioxide condenses out of the atmosphere as snow or frost, covering a region of the south pole.
Brush up on your Mars knowledge here.