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.

[Q]: What occurs if the body's renal system fails to work?  
 (A) Gas exchange is impaired. (B) Blood oxygenation no longer occurs. (C) Metabolic waste builds up in the blood. (D) Nutrients are no longer transported to organs.
[A]: C


[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.
[A]: D


[Q]: Which interaction between animals best controls the population of a species?  
 (A) wild animals marking their territories (B) remora fish traveling with sharks (C) lions preying on zebras (D) wolves traveling in packs
[A]:
C