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.

What gas in the air do we need to breathe in order to live?  
 (A) nitrogen (B) oxygen (C) carbon dioxide (D) hydrogen (E) water vapor
B

Which statement best describes the development of a human embryo?  
 (A) A single cell becomes many cells. (B) Each cell has a different function. (C) Many cells combine to form one cell. (D) All the cells have the same function.
A

Which of the following is an abiotic factor that can influence the quantity of fish a pond can support?  
 (A) availability of plants (B) concentration of bacteria (C) quality of water (D) number of consumers

C