Here's a problem to solve: In a lab investigation, students use Brand X batteries in a flashlight and Brand Y batteries in a radio. After two hours, the flashlight stops working, but the radio continues to work. As a result, the students conclude that Brand Y batteries last longer. Which statement about the conclusion of the investigation is most accurate?

Among the 4 following options, which is the correct answer?

- A: The conclusion is valid because they tested two different brands.
 
- B: The conclusion is invalid because the test had multiple variables.
 
- C: The conclusion is valid because they conducted the investigation in a lab.
 
- D: The conclusion is invalid because the real names of the batteries were hidden.
 