Given a scientific passage and an answer, generate a question for the given answer.

Q: Passage: Figure 29.17 The tails of the Hale-Bopp comet point away from the Sun, evidence that light has momentum. Dust emanating from the body of the comet forms this tail. Particles of dust are pushed away from the Sun by light reflecting from them. The blue ionized gas tail is also produced by photons interacting with atoms in the comet material. (credit: Geoff Chester, U. Navy, via Wikimedia Commons). Answer: momentum

A: The tails of the hale-bopp comet point away from the sun, evidence that light has what property of motion?
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Q: Passage: An ideal engine, the most efficient theoretically possible, is called a Carnot Engine. Its efficiency is given by the following formula, where the temperatures are, respectively, the temperature of the exhaust environment and the temperature of the heat input, in Kelvins. In a Carnot engine heat is input and exhausted in isothermal cycles, and the efficiency is . In all real engines heat is lost to the environment, thus the ideal efficiency is never even close to being obtained. Answer: carnot engine

A: What is the name of the the most efficient possible engine?
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Q: Passage: If only one atom of a specific type is present, no subscript is used. Answer: subscript

A:
If only one atom of a specific type is present what is not used?
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