We know that the temperature is well below selenium's boiling point, but because we do not know its melting point, we cannot be certain if the temperature has dropped low enough for the element to be a solid.

When matter changes from one physical state (solid, liquid, gas, or plasma) to a different physical state, a phase change is defined to have occurred.

When a solid changes to a gas, the phase change that occurred is defined as sublimation:

Solid  Gas

When the opposite occurs and a gas changes to a solid, the phase change is defined as deposition:

Gas  Solid

When a liquid changes to a gas, the phase change that occurred is defined as vaporization. It is worth noting that there are two types of vaporization: evaporation (when the phase change occurs below the boiling point for the substance) and boiling (when the phase change occurs at or above the substance's boiling point).

Liquid  Gas

When the opposite occurs and a gas changes to a liquid, the phase change is defined as condensation:

Gas  Liquid

When a liquid changes to a solid, the phase change that occurred is defined as freezing:

Liquid  Solid

When the opposite occurs and a solid changes to a liquid, the phase change is defined as melting:

Solid  Liquid

When a gas changes to a plasma, the phase change that occurred is defined as ionization:

Gas  Plasma

When the opposite occurs and a plasma changes to a gas, the phase change that occurred is defined as recombination.

Plasma  Gas

Some may find it easiest to remember the phase changes by memorizing them in pairs. For example freezing and melting both refer to phase changes between solids and liquids. Once that is memorized, the possible phase changes that you have to choose from is reduced from 8 to 2. Also, it may be helpful to think that all the phase changes involving a decrease in entropy (increase in order) deposition, condensation, freezing, and recombination sound like more orderly words than the phase changes involving an increase in entropy (decrease in order) sublimation, vaporization, melting and ionization.

This question is asking us about the effect that elevation has on the boiling point of water. In order to answer this, we need to have a fundamental understanding of what boiling is, and how it can be affected by factors such as elevation.

Boiling is a type of phase transition in which a liquid is converted into a gas. During this transition, the boiling point is defined as the temperature at which the vapor pressure of the liquid is exactly equal to the atmospheric pressure. Therefore, if the atmospheric pressure is lowered, which happens at increased elevations, then it becomes easier for the liquid to evaporate into the gas phase. Consequently, less energy (and a lower temperature) is needed to push the liquid into the gas phase. As a result, the boiling point of the solution is lowered at increased elevations.