Given the task definition and input, reply with output. 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.

Why is water from an aquifer more likely to be cleaner than water from other sources?
(A) because it forms where fresh and salt water meet (B) because it receives water directly from precipitation (C) because it rises to the surface near the ocean (D) because pollutants are filtered by rock and soil deep within Earth
D