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.

Many things can contribute to the development of a theory. A student made a list of some things that might help scientists develop a theory. Observations Calculations Conversations Publications From the student's list, which must be used in the development of every theory?  
 (A) Observations (B) Conversations (C) Publications and calculations (D) Calculations and conversations
A

Where do good bacteria grow to help keep people healthy?  
 (A) in the brain (B) in the blood (C) on the skin (D) on the bones
C

Down feathers are used by many sleeping bag manufacturers because down feathers are  
 (A) fire resistant. (B) comfortable padding. (C) good insulators. (D) water resistant.

C