speeds up the experiment. improves the reliability of the results. makes the graphs easier to read. ensures the students learn something.
I gave my students this multiple choice question: A student wants to determine which hand soap kills more bacteria. Her teacher tells her to change only the type of hand soap. The type of bacterium tested must be the same. Only one variable is tested because it

Only one answer is correct among these 4 choices:
- speeds up the experiment.
- improves the reliability of the results.
- makes the graphs easier to read.
- ensures the students learn something.

Could you tell me which one is correct?

improves the reliability of the results.