Q: 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.
Scientists go to a lake once a month to take samples of water. They are most likely studying the  
 (A) number of fish. (B) depth of the lake. (C) quality of the water. (D) speed of the water.
A:
C