Given scientific fact as input, generate the question from this fact such that it can be answered from the input.
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Question: A student should use a balance to compare the masses of two small rocks.

Answer: Which tool should a student use to compare the masses of two small rocks?


Question: Just as salty taste involves perception of sodium ions in saliva, sweet taste correlates with presence of glucose.

Answer: Just as salty taste involves perception of sodium ions in saliva, what taste correlates with presence of glucose?


Question: The brain stem monitor(s) the level of carbon dioxide in the blood.

Answer:
What monitors the level of carbon dioxide in the blood?