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]: A scientist makes a discovery while performing an investigation, but fails to maintain clear records of the tests performed. In which way does a lack of record keeping affect a scientist's work?  
 (A) It invalidates the procedures and conclusions. (B) It makes the data obtained in the study incorrect. (C) It prevents other scientists from validating the results. (D) It prohibits other scientists from developing new hypotheses.
[EX A]: C

[EX Q]: Which of these is an example of water changing from water vapor to liquid?  
 (A) moisture forming on a mirror when you breathe on it (B) sweat forming on your body when you exercise (C) ice cubes melting when you put them in a warm liquid (D) rivers drying up during a very hot summer
[EX A]: A

[EX Q]: When studying the health of an ecosystem, ecologists consider many factors. Which factor is least likely considered by ecologists when evaluating the impact of human activities on an ecosystem?  
 (A) air quality (B) plant diversity (C) solar radiation (D) thermal pollution
[EX A]:
C