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.

Q: Which situation is the best example of an inherited trait?  
 (A) a boy playing with a ball (B) a bird building a nest (C) a horse pulling a wagon (D) a dog sitting on command

A: B
****
Q: Which organ is the site of gas exchange between the body and the environment?  
 (A) spleen (B) liver (C) lungs (D) pancreas

A: C
****
Q: Mammals must eliminate waste products that their bodies produce. Which organ helps mammals eliminate bodily waste?  
 (A) appendix (B) kidney (C) pancreas (D) stomach

A:
B
****