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]: 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.
[EX A]: D

[EX Q]: Which factors can have the greatest effect on the health of a river system?  
 (A) type of soil and salinity (B) nitrate levels and turbidity (C) human consumption and pH (D) natural disasters and tidal changes
[EX A]: B

[EX Q]: In Australia, mid-day temperatures can soar to over 110°F (43°C). In order to keep their bodies cool, kangaroos lick their arms, coating them with a thick layer of saliva. As the saliva evaporates, it acts as a cooling mechanism and lowers the kangaroo's body temperature. Which term best describes this activity?  
 (A) natural selection (B) defense mechanism (C) structural adaptation (D) behavioral adaptation
[EX A]:
D