Astudent is measuring the effects of an electric circuit on a compass, and he is testing to see if increasing the current will cause the compass needle to change its 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? he should repeat the test with three additional batteries of varying voltages to demonstrate the reproducibility of results. he should use batteries of the same voltage but from different manufacturers, to show he has used different variables. he should add a nonworking compass to act as a control. he should use only one battery but do several trials, using different circuit designs to ensure he has appropriate test groups.