instruction:
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.
question:
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
answer:
B


question:
The fur of some animals becomes thicker in the winter and is shed in the spring. These observations show how animals  
 (A) respond to changes in the environment (B) move from place to place to survive (C) store fat for the winter (D) compete with other animals for food
answer:
A


question:
From year to year, farmers rotate different crops in the fields to improve soil nutrients. Why is crop rotation also an effective pest management method?  
 (A) It allows chemicals to kill more pests. (B) It creates crops that are pest-resistant. (C) It interrupts the life cycles of pests. (D) It allows pests to overpopulate.
answer:

C