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Good job! As you may remember, you can display the levels of a factor vector, like this:


Recall what we said before about NA—it represents a missing value. Much like it doesn't make sense to perform operations on missing values, it also doesn't make sense to define levels based on missing values! Thus, NA is not considered a level.


The air_conditioner factor you created in the previous exercise contains some missing values. Display the levels of this factor. Is NA a level?