What is true of the maximum possible efficiency of a heat engine?

Question

A. It is 100%.
B. It depends on the difference between the exhaust and intake temperatures.
C. It depends on the ratio between the exhaust and intake temperatures.
D. It depends on the intake temperature.

Answer 1

A heat engine works on a difference in heat. Heat always flows from a hotter to a cooler place. A heat-engine taps some of that energy flowing from hot to cold and turns it into useful energy.
Heat is the lowest form of energy; all energy eventually becomes heat. And heat itself cannot be used as an energy-source. Only heat-difference can be used as an energy-source.

A. Wrong. It can't be 100% because that means you've removed ALL the energy from the heat. It's not possible to do that.
B. Right.
C. Wrong.
D. No, this answer doesn't mention the heat-difference.

B is right because if your input is, say, 200 degrees and the output is 100 degrees, the difference (100 degrees) is what makes the engine run. The ratio here is 1:2 but the temperature-difference is 100 degrees.
The engine would run the same if you fed it with 500 degrees and cooled it with 400 degrees. Although the ratio is very different, the engine runs exactly as fast.
Therefore B is right and C isn't.

Answer 2

The answer is C.it depends on the ratio between the exhhaust(out let) and intake( in let) temperatures.

Answer 3

B