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]: Which characteristic of frogs is an adaptation to living in water?  
 (A) strong legs (B) webbed feet (C) dull colors (D) moist skin
[EX A]: B

[EX Q]: What property is the same for each atom of an element?  
 (A) energy (B) mass number (C) atomic number (D) number of neutrons
[EX A]: C

[EX Q]: Which is the thickest layer of Earth?  
 (A) crust (B) inner core (C) mantle (D) outer core
[EX A]:
C