You are given a science question (easy-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]: Scientists have studied the specific stages in the life cycle of stars. In which stage do stars spend most of their lifetime?  
 (A) red giant (B) black dwarf (C) white dwarf (D) main sequence
[EX A]: D

[EX Q]: Which best describes a characteristic of the jet stream?  
 (A) It forms a boundary between a cold air mass and a warm air mass. (B) It creates the high winds around the eye of a hurricane. (C) It forces the hot air along the equator to rise to areas in North America. (D) It causes high pressure air masses to rotate clockwise.
[EX A]: A

[EX Q]: The bacterium Brucella abortus causes fever in humans. Biologists studying this bacterium found that when exposed to light, the bacterium becomes more virulent. How can this change in virulence most likely be described?  
 (A) as a behavioral adaptation (B) as a survival disadvantage (C) as a response to a stimulus (D) as an acquired characteristic
[EX A]:
C