An experiment is designed to test what color of light will activate a photoelectric cell the best. The photocell is set in a circuit that "clicks" in response to current. The faster the current, the more clicks per minute. In this experiment, the number of clicks in one minute is recorded for each color of light shining on the photocell. To change the color of light, a different color of cellophane is placed over the same flashlight and the flashlight is then located a specific distance from the photocell. Which of the following was not kept constant?

the test for photocell activation
the color of the light after it has passed through the cellophane
the distance of the light from the photocell
the source of the original light