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

Which land feature is formed from erosion by waves?   
 (A) a sandbar (B) a sea cliff (C) a barrier reef (D) a barrier island
B

Which of the following has changed over the last 200 years primarily due to human activity?  
 (A) Earth's magnetic field (B) the frequency of earthquakes (C) Earth's tectonic plate activity (D) the composition of the atmosphere

D