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.

[Q]: The human immunodeficiency virus (HIV) causes AIDS. The first known human cases AIDS were in Africa in the late 1950s. Since then, tens of millions of people have died from the disease, and tens of millions more people all over the world are infected. Which term best describes the disease AIDS?
(A) isolated cases (B) outbreak (C) epidemic (D) pandemic
[A]: D

[Q]: The pull of gravity from Earth has the greatest effect on
(A) comets. (B) the Sun. (C) asteroids. (D) the Moon.
[A]: D

[Q]: To calculate the relative age of rocks, geologists use the rate of radioactive decay of isotopes present in their samples. What is the term used for the original amount of the elemental isotopes being investigated?
(A) rare earth isotopes (B) son isotopes (C) daughter isotopes (D) parent isotopes
[A]:
D