Gretchen wants to run a test to discover whether glass, copper, wood, or plastic conduct electricity. She sets up a circuit connecting a battery to a light bulb, with a small gap she will use to test the conductivity of the different substances. One at a time, she connects the circuit with the chosen test material, and records whether or not the light bulb comes on. The variables in this experiment are

A) The wood and glass.
B) The copper and plastic.
C) The light bulb and battery.
D) The plastic, wood, copper, and glass.