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.

[EX Q]: The systems of the human body work together. Which system works with the skeletal system by pulling on the bones to create movement?  
 (A) digestive (B) circulatory (C) muscular (D) nervous
[EX A]: C

[EX Q]: The excessive use of nitrogen-based fertilizers causes runoff that increases the nitrogen content of lakes and bays. These nitrogen compounds directly affect the aquatic ecosystems by  
 (A) interfering with the water cycle. (B) decreasing the predator population. (C) increasing the rate of algae growth. (D) causing aquatic animals to grow larger.
[EX A]: C

[EX Q]: Proper use and care of a microscope includes  
 (A) carrying it with both hands. (B) reflecting sunlight to the stage. (C) removing the objectives after you are done. (D) adjusting the coarse focus under high power.
[EX A]:
A