to act as a controlled variable for the investigation to act as the dependent variable for the investigation to provide another testable variable for the investigation to serve as the independent variable for the investigation
I gave my students this multiple choice question: A student performs an investigation using anti-bacterial hand soap. The student washes only his left hand with anti-bacterial soap, but cultures swabs from both hands. The cultures from both hands were incubated at room temperature for 48 hours before being examined. Which best explains the purpose of using soap on only the left hand?

Only one answer is correct among these 4 choices:
- to act as a controlled variable for the investigation
- to act as the dependent variable for the investigation
- to provide another testable variable for the investigation
- to serve as the independent variable for the investigation

Could you tell me which one is correct?

to serve as the independent variable for the investigation