TASK DEFINITION: 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.
PROBLEM: 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.

SOLUTION: C

PROBLEM: Which factor would most likely limit the number of mice living in an area?  
 (A) plentiful resources (B) stable climate (C) more predators (D) less competition

SOLUTION: C

PROBLEM: Which of these is a mixture?  
 (A) salt water (B) sugar (C) water vapor (D) salt

SOLUTION:
A