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.

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.

A: D
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Q: Wave energy from the ocean can be harnessed to power generators to make electricity. Energy from ocean tides can also be used to make electricity. How would you categorize these two sources of energy?  
 (A) Both are renewable. (B) Both are nonrenewable. (C) Wave energy is nonrenewable. Tidal energy is renewable. (D) Wave energy is renewable. Tidal energy is nonrenewable.

A: A
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Q: It is necessary to add a day to the calendar every four years because  
 (A) the axis of Earth is tilted. (B) the gravitational pull of the Sun affects Earth's revolution. (C) the revolution of Earth is not exactly 365 days. (D) the Moon crosses the orbit of the Sun every 28 days.

A:
C
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