Problem:OPTIONS:
- standing water
- coastlines
- mouth of rivers
- ice wedging
- liquid precipitation
- Physical weathering
- trees
- gravity

You are presented with the quiz "what causes erosion?" But you don't know the answer, so you turn to your teacher to ask for hints. He says that "rain causes erosion" and "rain is liquid precipitation". So, what's the best answer to the question?
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A: liquid precipitation

Problem:OPTIONS:
- fed by hand
- homogenous
- fed by others
- parasites
- fed red meat
- fed whole grains
- important habitats
- insects

You are presented with the quiz "All animals are what?" But you don't know the answer, so you turn to your teacher to ask for hints. He says that "all animals are heterotrophs" and "heterotrophics means fed by others". So, what's the best answer to the question?
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A: fed by others

Problem:OPTIONS:
- resistance activities
- genetic ties
- genetic distractions
- Plant reproduction
- fur and fat
- resistance
- genetic variations
- genetic similarities

You are presented with the quiz "Most what in a gene pool are beneficial to organisms?" But you don't know the answer, so you turn to your teacher to ask for hints. He says that "mutation creates new genetic variation in a gene pool" and "most mutations are beneficial to the organism". So, what's the best answer to the question?
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A: genetic variations

Problem:OPTIONS:
- a cuticle
- storing system
- city system
- epidermis and dermis
- nervous system
- building system
- four limbs
- layers of fat

You are presented with the quiz "most insects have a what?" But you don't know the answer, so you turn to your teacher to ask for hints. He says that "most invertebrates have a nervous system" and "insects are the most common invertebrates". So, what's the best answer to the question?
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A:
nervous system