B
Below is a question with multiple-choice answers. Choose the correct option based on your reasoning.

### Question:
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?

### Choices:
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.

### Answer:
