instruction:
Given scientific fact as input, generate the question from this fact such that it can be answered from the input.
question:
Unlike ammonia, oxygen cannot be liquefied at room temperature because its critical temperature is below room temperature.
answer:
Unlike ammonia, oxygen cannot be liquefied at room temperature because its what is below room temperature?

question:
Wave amplitude and wavelength are two important measures of wave size.
answer:
Wave amplitude and wavelength are two important measures of what?

question:
Salicylic acid do willow trees produce to kill bacteria.
answer:
What do willow trees produce to kill bacteria?