Ella set up an experiment to investigate one of her questions. She set up one test tube to look like Figure 1. She set up another test tube similar to Figure 1 but without
a lamp, and placed it in a dark closet. A day later, Ella checked the water level in
each test tube.

What was the test (independent) variable in the student's experiment?
A. The amount of light.
B. The amount of time.
C. The water level in the test tube.
D. The amount of oxygen released.