An Experimenter Served Identical Meals to People at Six Tables

An experimenter served identical meals to people at six tables at an elegant restaurant.At each table there was a small lantern with a rheostat.Each table had a different brightness of lighting.There was a direct correlation between the time it took the people to finish their meals and the amount of artificial light from the lantern.The dependent variable in this experiment was:

A) the degree of light.
B) the personal eating habits of the subjects.
C) experimenter bias in establishing guidelines.
D) the length of time it took to finish the meal.