Question

1. the fuel tank of an airplane has a device that measures the fuels temperature. before the airplane took off, the temperature of the fuel was 27°c. after the airplane had been flying for one hour, the temperature of the fuel had decreased to 3°c. which of the following must have happened to the molecules of the fuel?

a. the molecules got farther apart.

b. the molecules started moving slower.

c. the fuel gained new cold molecules.

d. the molecules became smaller in size.

Explanation:

Temperature is a measure of the average kinetic energy of molecules. When temperature decreases, the average kinetic energy of the fuel molecules drops, meaning they move slower. Option a is incorrect because cooling a liquid/gas (fuel) would bring molecules closer, not farther. Option c is wrong because there are no "cold molecules"; temperature is about molecular motion, not a type of molecule. Option d is incorrect because molecular size does not change with temperature.

Answer:

b. The molecules started moving slower.