The control in an experiment is the standard against which the results of the experimental group are compared. In this case, the control group would be the standard condition that the student uses as a reference point for comparison with the other groups. Based on the information provided, it is not clear what the student used as a control group. However, option B, "the resistances that were measured," is not the control group because it is the dependent variable being measured and not the control.
Option C, "the widths of the wires," is a possible choice for the control because it is the independent variable that is being manipulated to test its effect on the dependent variable (resistance). Option D, "the wires are all copper," could also be considered as a possible control because it ensures that the material properties of the wires are consistent across all groups, eliminating any potential material-related confounding variables. Therefore, the best answer is A, "All of these," as all of these factors may have contributed to serving as the control in this experiment.