Problem:

You are presented with the quiz "What is used for heating buildings?"   But you don't know the answer, so you turn to your teacher to ask for hints. He says that "electricity sometimes is used for heating buildings" and "electrical energy is also energy".   So, what's the best answer to the question?
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A: energy


Problem:

You are presented with the quiz "cycles of freezing and thawing water can cause what?"   But you don't know the answer, so you turn to your teacher to ask for hints. He says that "cycles of freezing and thawing water cause ice wedging" and "an ordinary ice road is therefore severely damaged by cracks induced by the traffic".   So, what's the best answer to the question?
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A: severely damaged roads


Problem:

You are presented with the quiz "having what is a cause of mortality"   But you don't know the answer, so you turn to your teacher to ask for hints. He says that "having no natural predators increases population size" and "population size is a function of natality and mortality".   So, what's the best answer to the question?
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A: natural predators


Problem:

You are presented with the quiz "what requires water for survival?"   But you don't know the answer, so you turn to your teacher to ask for hints. He says that "aplants require water for survival" and "all plants are of bush type".   So, what's the best answer to the question?
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A: