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. is this a good experimental design? a) no, because she is testing too many things at once. b) no, since we don't know the results of the experiment. c) yes, because all of the materials gave the results she predicted. d) yes, because she is only changing one piece of the experiment at a time.