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]: Several horses grazed in a fenced area across from a home. On rainy days, soil would wash down a slope and run toward the home. After the horses were moved a few years later, the soil no longer washed down when it rained. What could account for this change?  
 (A) The grass grew and kept the soil intact. (B) The fence kept the soil contained. (C) The soil was completely gone. (D) The amount of rain decreased.
[EX A]: A

[EX Q]: Which factors can have the greatest effect on the health of a river system?  
 (A) type of soil and salinity (B) nitrate levels and turbidity (C) human consumption and pH (D) natural disasters and tidal changes
[EX A]: B

[EX Q]: The human skin plays an important role in a variety of organ systems. Which organ system is least likely to have a direct interaction with the skin?  
 (A) digestive (B) excretory (C) immune (D) nervous
[EX A]:
A