instruction:
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.
question:
Which characteristic best identifies a pure substance?  
 (A) shape (B) texture (C) melting point (D) state of matter
answer:
C


question:
Which of these will most likely harm a habitat?  
 (A) planting trees (B) water pollution (C) rainfall (D) sunlight
answer:
B


question:
The complete removal of decomposers from an ecosystem will have the greatest effect on which of the following?  
 (A) the spread of disease (B) the availability of water (C) the recycling of nutrients (D) the distribution of organisms
answer:

C