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I have a question about a practice test!! Why is the correct answer D? I chose C.

The basic operation behind this system uses the heat energy stored in the oceans as a source of power. The plant exploits the difference in water temperature between the warm surface waters heated by the sun and the colder waters found at ocean depths. A minimum temperature difference of 20 degrees Celsius between surface and depth is required for efficient operation, and this situation is typically found only in tropical and subtropical regions of the world. There are two basic kinds of OTEC system: the open cycle system and the closed cycle system. In the open cycle system, the warm surface water is converted into steam in a partial vacuum and this steam drives a turbine connected to an electrical generator. In a closed cycle system, the warm surface water is used to boil a fluid, such as ammonia, which has a low boiling point. In both systems cold water pumped up from the ocean depths condenses the vapor. In the open system, the steam is condensed back into a liquid by cold water pumped from deep-ocean water and then discharged. In the closed system, the condensed ammonia is used to repeat the cycle continuously. Various hybrid systems using characteristics of both open and closed cycle plants have also been designed.
According to the passage, what can be inferred about the factor that allows the ocean to be used as an energy source?
A. The oceans are so large that they can produce a lot of energy.
B. In polar climates, the sun does not sufficiently heat the deeper water for practical energy use.
C. The oceans can store vast amounts of heat energy to be used to run basic electricity plants.
D. The plants are typically found in the tropical and subtropical regions of the world because of the warm weather.
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1 Answer

C is technically true; however, it doesn't actually tie in with the process the article describes.  In the article, the focus is on the difference between temperatures at different depths in the ocean, not the simple presence of heat energy.  To run through the answers one by one:
A is fairly obviously incorrect.  While it's true, it has nothing to do with this article.  The energy produced by the oceans is largely mechanical, while the text focuses on heat.
B is tempting.  It discusses the difference in temperatures that the article goes into so much detail on.  A deep reading can catch this one, though - when it describes the process, the article talks about "cold water pumped up from the ocean depths," and this answer says that deep water being cold is a problem.  So, incorrect, but it's one of the "trick" answers.
C is like A: true but irrelevant.  The point of the article is not that the oceans have a large amount of energy stored away but rather that there are a couple of methods for getting at that energy.  Both of those methods use a temperature differential to run the plants, not just raw energy from the water, so C doesn't actually say anything meaningful.  Yes, it's almost the same as one of the first sentences in the article, but that doesn't make it the main "factor that can be used as an energy source."  That's the difference, not the simple amount.
D is the only one left, but it's good to look over it first.  The article never mentions tropical regions at all, but it constantly mentions warm shallow water.  This question expects you to use some outside knowledge, specifically that shallow water tends to stay close to the local temperature while deep water is almost always cold.  Since the plants need warm water at the surface and cold water below, that makes tropical areas the best place to build them.  Again, it's not written in the article anywhere, but that's why the question asks you to "infer."
This was definitely a tricky question considering it had two very tempting wrong answers - and, for that matter, that all three wrong answers were still true to some degree.  Be careful not to simply select the answer that seems to be written in the text, because that's frequently bait to stop you from seeing the actual focus.
I hope this helps!


Hello, Jesse!!
Thank you for your help!! This is a tricky question, and I have to guess from the passage.
So, its difficult for me to choose a correct answer. But, now I understand.
The author mentions warm shallow water,and also explains the difference in water temperature.
So, I have to infer from few information little information!!