The director is wrong. the lurking variable here is the temperature.

a lower temperature leads to a more severe blizzard, which calls for more plows and keeps people at home, where they are more likely to make online purchases.
b. the director is wrong. if more plows are sent out whenever it snows, the roads will be better and people will leave their home to make purchases, as opposed to staying home and increasing online sales.
c. the director is wrong. the lurking variable here is the severity of the blizzard.
d. a more severe blizzard calls for more plows and keeps people at home, where they are more likely to make online purchases.