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.

Ex Input:
Which set contains kingdoms that contain only heterotrophs?
(A) Protists, Fungi (B) Bacteria, Animals (C) Plants, Fungi (D) Animals, Fungi

Ex Output:
D

Ex Input:
Two identical books are on opposite ends of a table. Which of these would increase the gravitational force exerted between the two books?
(A) decrease the distance between the books (B) decrease the mass of the books (C) decrease the mass of the table (D) decrease the temperature in the room

Ex Output:
A

Ex Input:
Individuals that carry a particular allele may have more descendants than other individuals, resulting in the allele becoming more common. Which process causes random alleles to become more common within a population?
(A) gene shuffling (B) directional selection (C) natural selection (D) genetic drift

Ex Output:
D