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.

[EX Q]: A potential negative impact of building a dam on a river is that the dam   
 (A) prevents sediment from flowing downstream. (B) increases the amount of water available to farms. (C) prevents seasonal downstream flooding. (D) increases the rate of water loss from a lake.
[EX A]: A

[EX Q]: Which example best shows structures that make up the digestive system in the human body?  
 (A) heart, blood, vessels (B) nose, trachea, lungs (C) brain, spinal cord, nerves (D) esophagus, stomach, intestines
[EX A]: D

[EX Q]: Ancient Greeks hypothesized that a hair from a horse's tail left in water would turn into a worm. What scientific process most likely would have disproved this hypothesis?  
 (A) forming a theory (B) designing a model (C) analyzing data (D) comparing to a control
[EX A]:
D