You are given a science question (hard-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.

Example Input: Many horses grow a heavy coat in autumn and shed that coat in spring. Scientists were not sure if temperature or the amount of daylight per day (called the photoperiod) caused the change. So, they conducted an experiment and concluded that the change in photoperiod was responsible for the biological changes. Shedding caused by which set of conditions would have helped them arrive at this conclusion?  
 (A) a constant photoperiod, but varied temperatures (B) a varied photoperiod, and varied temperatures (C) a constant photoperiod, and a constant temperature (D) a varied photoperiod, but a constant temperature
Example Output: D

Example Input: What part of the digestive system first causes chemical changes to food?  
 (A) teeth in the mouth (B) saliva in the mouth (C) enzymes in the stomach (D) enzymes in the small intestine
Example Output: B

Example Input: Felipe made a visual model of an estuary ecosystem on a poster. He colored the ocean water blue and the river water yellow. Which would be the best color for him to use to visually represent the estuary?  
 (A) yellow (B) blue (C) orange (D) green
Example Output:
D