Part 1. Definition
You are given a science question (hard-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.
Part 2. Example
Which event can cause a large rock to become soil over time? 
 (A) decaying (B) an earthquake (C) a landslide (D) weathering
Answer: D
Explanation: Under the action of heat, cold, rain, wind, and other atmospheric factors, the rock breaks down physically into small fragments that become the parent material of the soil. Also, atmospheric factors change according to weather.
Part 3. Exercise
Which is the best explanation for how air masses move across the United States?  
 (A) The prevailing westerlies move air masses from west to east across the United States but may be deflected by the jet stream. (B) The trade winds move air masses from west to east across the United States. (C) The jet stream moves air masses from the Pacific Ocean across the United States. (D) The warm air of the Gulf Stream causes air masses to move from the Atlantic Ocean to the Pacific Ocean.
Answer:
A