Final answer:
Moderate temperatures allow Earth to hold onto liquid water.
Step-by-step explanation:
The factor that allows Earth to hold onto liquid water is the moderate temperatures. The presence of living things is not the factor, as water existed on Earth even before the emergence of life. Lunar tides are also unrelated to Earth's ability to hold liquid water. Lack of gravity is not the correct factor either, as Earth has gravity which helps to hold the water in its atmosphere and on its surface.
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