The answer is D.rainforest. it is a very complex ecosystem with variety of plants and animals.
Electron pairs , because the atoms become stable by gaining the electron pairs
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B. some substances must be dissolved in water before they can be used
Think about it. Caves are dark, with little to no light. If there's no light, why would you need to see? There are far more important senses than sight in cases like these. Senses such as smell, hearing, and touch.
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