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]: The visible colors listed from shortest to longest wavelength are violet, blue, green, yellow, orange, and red. Which of the following accurately compares the frequency of these colors using this information?  
 (A) Blue has a higher frequency than violet. (B) Blue has a higher frequency than green. (C) Orange has a lower frequency than red. (D) Violet has a lower frequency than yellow.
[EX A]: B

[EX Q]: A pharmaceutical company developed a drug to help relieve headaches. Recently, the pharmaceutical company discovered that the drug may cause nausea in a small number of people who might take the drug. Which is the most ethical course of action that the drug company could take?  
 (A) Discontinue the use of the drug. (B) Inform people of the possible side effects. (C) Reduce the amount of drug given to people. (D) Modify the drug to include medicine that prevents nausea.
[EX A]:
B