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 of these is a structure that is present in all organisms?  
 (A) a tissue (B) a cell (C) a stomach (D) a brain
[EX A]: B

[EX Q]: Diamonds are sometimes used as the tips of drill bits. Which physical property of a diamond MOST likely makes it a good cutting tool?  
 (A) its electrical properties (B) its thermal properties (C) its hardness (D) its density
[EX A]: C

[EX Q]: Some changes are fast. Some changes are slow. Which of these changes is most likely the slowest?  
 (A) fire (B) flood (C) landslide (D) drought
[EX A]:
D