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.

In the human body, which system functions primarily to defend the body against disease?  
 (A) digestive (B) immune (C) nervous (D) respiratory
B

How do traits of animals such as ear shape, nose shape, and hair color most often get passed to offspring?  
 (A) sexual reproduction (B) asexual reproduction (C) adaptation (D) instinct
A

What is the smallest unit of an element that retains the properties of that element?  
 (A) electron (B) molecule (C) neutron (D) atom

D