The correct answer is D. Decrease in atmospheric pressure
Step-by-step explanation:
The boiling point of substances is the temperature in which a liquid substance begins to change to a gaseous state. This temperature is affected by the elevation because at higher altitudes the boiling point is lower. For example, at around 9,000 feet the boiling point of water is around 90 °C instead of 100 °C.
This can be explained as the altitude is directly related to the atmospheric pressure (pressure of the atmosphere on the surface) which means at higher elevation the atmospheric pressure decreases. At the same time, less atmospheric pressure leads to less heat to get to the boiling point because the atmospheric pressure reduces the space of particles in a liquid and therefore stops them from changing its state. This means, with a decrease in atmospheric pressure liquids require less heat to boil. Thus, the change in boiling temperature at 9000 feet is due to a decrease in atmospheric pressure.