Correct Response: D. Changing the values of two factors at the same time is the most significant problem with this experimental design. Limiting the change to one variable at a time is important because it isolates the effect of each variable. Not accounting for the resistance of the additional length of wire (A) would most likely not present a problem for this experiment, since the field strength would increase as the number of turns increases, even though the resistance of the coil also increases. Using paper clips to measure the strength of the electromagnet (B) is not a problem, since a stronger magnetic field can lift more paper clips than can a weaker magnetic field. Using increments of 2.0 V instead of 1.0 V (C) does not present a problem as long as the wire can withstand the higher currents and as long as the voltage is manipulated while the number of turns and any other variables are held constant.
