start the experiment at the same time. have different members record results. use the same equipment and procedures. perform a different number of trials.
I gave my students this multiple choice question: In order to compare the results of an experiment among different groups, each group must

Only one answer is correct among these 4 choices:
- start the experiment at the same time.
- have different members record results.
- use the same equipment and procedures.
- perform a different number of trials.

Could you tell me which one is correct?

use the same equipment and procedures.