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.
--------
Question: A population of plants growing on an island consisted of two varieties, one with thorns and the other without. Over a period of many years, the variety with thorns gradually disappeared. Which process most likely brought about this change in the plant population?
(A) gene flow (B) genetic drift (C) natural selection (D) random mutation

Answer: C

Question: Why are some rocks smoother and rounder than others?
(A) They have the shapes of the minerals from which they are made. (B) They were smoothed by the action of wind and water. (C) They were squeezed up through the soil. (D) They hardened in rounded pockets of soil.

Answer: B

Question: When the pesticide DDT was first used, it killed nearly every mosquito it touched. Within a few years, however, many mosquitoes became resistant to DDT and survived. What enabled this to happen?
(A) meiosis (B) migration (C) immune responses (D) gene mutations

Answer:
D