TASK DEFINITION: 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.
PROBLEM: A scientist combines oxygen and hydrogen to form water. This combination illustrates that water is  
 (A) an atom. (B) an element. (C) a mixture. (D) a compound.

SOLUTION: D

PROBLEM: 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

SOLUTION: C

PROBLEM: Water enters a plant cell through the  
 (A) cell wall. (B) chloroplasts. (C) mitochondria. (D) nucleus.

SOLUTION:
A