Mass number is O the average atomic mass of an element.
O the total number of electrons in an atom of an element.
O the total number of protons in an atom of an element.
O the total number of protons and neutrons in an atom of an element.

4 th is the answer

if the temperature drops and becomes cool

then the atmosphere can be freezing as the water vapor in the air is at maximum (100%) and can be severely cold

also can be snowing lightly as the moisture in the air will start to solidify

and btw this is not chemistry but