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

The early Greeks are credited with many valid concepts in astronomy. Some of their theories were correct; some were later proven incorrect. One theory was that Earth was the center of the universe and that other planets circled Earth. The Greeks thought Earth did not move because its movement was not obvious from the surface of the planet. The Greeks also believed that an invisible sphere surrounding our planet contained the stars. This sphere rotated, explaining the apparent movement of constellations over time. Early Greeks also developed theories about the relationship between the moon and Earth. Why is the same side of the moon always visible from Earth?
(A) The far side of the moon does not reflect sunlight. (B) The moon stays in one location while Earth revolves around it. (C) The period of rotation and the period of revolution for the moon are about the same. (D) The moon rotates clockwise on its axis while Earth rotates counterclockwise on its axis.
C