Task: Question Answering
Input (English): 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?  
 (A) to act as a controlled variable for the investigation (B) to act as the dependent variable for the investigation (C) to provide another testable variable for the investigation (D) to serve as the independent variable for the investigation
Output (English): D