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: The pressure of a sample of gas is measured with an open-end manometer.

Answer: The pressure of a sample of gas is measured with an open-end what?


Question: Millibar is the unit used to measure air pressure.

Answer: What is the unit used to measure air pressure?


Question: Certain characteristics are frequently inherited together because of linkage.

Answer:
Certain characteristics are frequently inherited together because of what?