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]: After observing several dogs running, students ask the following question: Do dogs with long hair run faster than dogs with short hair? How can the students best answer their question?
(A) record the weights and heights of many dogs (B) measure the speeds and hair lengths of many dogs (C) research to find the type of dog with the longest hair (D) race one long-haired dog against one short-haired dog
[EX A]: B

[EX Q]: The Sun is classified as a star. Which characteristic identifies the Sun as a star?
(A) The Sun spins on its axis. (B) The Sun is located outside the Milky Way galaxy. (C) The Sun is part of the Big Dipper. (D) The Sun produces light and energy in a nuclear reaction.
[EX A]: D

[EX Q]: A sample of an unknown substance has a volume that is independent of its container. Which additional information is needed to determine the state of the sample?
(A) the mass of the sample (B) the density of the sample (C) whether the sample has a fixed shape (D) whether the sample conducts electricity
[EX A]:
C