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?

What is the correct answer to the question from the following choices?
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.;
(B).