You are given a science question (hard-level) and four answer options (associated with "A", "B", "C", "D"). Your task is to find the correct answer based on scientific facts, knowledge, and reasoning. Do not generate anything else apart from one of the following characters: 'A', 'B, 'C', 'D'. There is only one correct answer for each question.

[EX Q]: 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?  
 (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.
[EX A]: B

[EX Q]: What is the best practice to follow when completing an experiment?  
 (A) using caution when handling hot glassware (B) turning off burners when you are not using them (C) studying lab procedures prior to conducting the experiment (D) checking with your teacher on the procedure to dispose of chemicals
[EX A]: D

[EX Q]: Organisms have many methods of surviving winter conditions. Which of the following is NOT an example of an organism adapting to survive the cold winter months?  
 (A) Robins fly from Michigan to Georgia in the fall. (B) Chameleons change color to match their surroundings. (C) White-tailed deer grow longer hair that is hollow inside. (D) Carp settle on the bottom of a pond and cover themselves with mud.
[EX A]:
B