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: Many scientists are convinced that an increase of certain gases in the atmosphere is affecting global temperatures. What makes this hypothesis difficult to prove?
(A) Trees absorb these gases. (B) Global temperatures are rising. (C) Many variables affect global temperatures. (D) Many human activities produce these gases.

SOLUTION: C

PROBLEM: Which wind-water interaction is responsible for cycling nutrient-rich waters from the ocean floor to the sea surface?
(A) upwelling (B) storm surge (C) downwelling (D) surface currents

SOLUTION:
A