Astudent is measuring the effects of an electric circuit on a compass, and he is testing to see increasing the current will cause the compass needle to change direction faster. he does one trial using a 9-volt battery and another trial using a 1.5 volt battery. he takes accurate data. based on good experimental design, which would improve his experiment? a. he should use batteries of the same voltage but from different manufacturers to show he has used different variables. b. he should add a nonworking compass to act as a control. c. he should repeat the test with three additional batteries of varying voltages to demonstrate the reproducibility of results. d. he should use only one battery but do several trials using different circuit designs to ensure he is appropriate test groups. ( i think it's a or c)