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]: Which is the best unit to measure distances between Earth and other solar systems in the universe?  
 (A) miles (B) kilometers (C) light years (D) astronomical units
[EX A]: C

[EX Q]: Which is a hypothesis that can be easily tested?  
 (A) The Moon causes high tides. (B) During which season are the tides the highest? (C) What is the phase of the Moon when tides are highest? (D) When the Moon is full, the tides will be at their highest.
[EX A]: D

[EX Q]: Beavers build their homes in ponds and streams. Which characteristic is least critical to building homes in an aquatic environment?  
 (A) waterproof fur (B) webbed hind feet (C) large, sharp teeth (D) flat, wide tail
[EX A]:
C