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.
One example is below.
Q: Which event can cause a large rock to become soil over time? 
 (A) decaying (B) an earthquake (C) a landslide (D) weathering
A: D
Rationale: Under the action of heat, cold, rain, wind, and other atmospheric factors, the rock breaks down physically into small fragments that become the parent material of the soil. Also, atmospheric factors change according to weather.
Q: A scientist investigated the effect of workplace stress on heart disease in humans. Men of various ages were divided into two groups based on whether they described their work as very stressful or not very stressful. During the one year investigation the scientist monitored the heart health of each man. What was the bias in this investigation?  
 (A) The investigation only lasted one year. (B) The only organ studied was the heart. (C) The investigation tested only men. (D) The age of the participants varied.
A:

Show your work and conclude with the exact phrasing "Therefore, the final answer is [answer]." where [answer] is just the final number or expression that solves the problem.
user
C