During a phase change, mass

a: stays the same
b: increases
c: decreases
d: increases, then decreases

Stays the same, within a closed system.

Your teacher has probably used this example before. When you heat water in a pot on the stove, as it starts to boil, the water evaporates and turns into steam. 
If you have two pots that are both heating on the stove, filled to the same water level; one is covered with an air-tight lid, and the other is left open.The water level in the open pot will be lower, because more steam was able to escape during a gaseous phase. 

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