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 Q]: There is a river at the base of Mount Tom. In the spring the water level is usually very high. In the summer it is lower. The higher level of water during the spring comes from  
 (A) hurricanes. (B) melting snow. (C) tornadoes. (D) evaporating water.
[EX A]: B

[EX Q]: Which of the following best describes an advantage of using a mass production manufacturing system instead of a custom manufacturing system?  
 (A) Customers can provide specific feedback to workers. (B) Workers become skilled in all aspects of assembly. (C) Goods can be easily modified for customers. (D) Products can be made at a lower cost.
[EX A]: D

[EX Q]: The basic units of structure and function for both plants and animals are  
 (A) cells. (B) organs. (C) systems. (D) tissues.
[EX A]:
A