when operating between temperatures 
and 
(Any reversible heat engine operating between those temperatures must have the same efficiency.) [P7.4]
is: a reversible heat engine that utilizes the Carnot cycle. [P7.4]
has: efficiency when operating between temperatures and (Any reversible heat engine operating between those temperatures must have the same efficiency.) [P7.4]
is: the most efficient possible heat engine operating between two fixed temperatures. [P7.4]
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