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 involves the greatest variety of cell types?  
 (A) a tissue (B) an organ (C) an organ system (D) an organism
[EX A]: D

[EX Q]: Some people carry a mutated allele of a particular gene that makes them resistant to HIV infection and AIDS. Which best describes this type of mutation?  
 (A) beneficial (B) asexual (C) harmful (D) unicellular
[EX A]: A

[EX Q]: Which part of an atom is involved in chemical reactions?  
 (A) nucleus and its particles (B) negatively-charged particles (C) neutrally-charged particles (D) positively-charged particles
[EX A]:
B