Some species of sharks grow to an enormous size, often weighing from one to four thousand pounds each. The skin of the shark is rough, and is used for polishing wood, ivory, &c.; that of one species is manufactured into an article called shagreen: spectacle-cases are made of it. The white shark is the sailor's worst enemy: he has five rows of wedge-shaped teeth, which are notched like a saw: when the animal is at rest they are flat in his mouth, but when about to seize his prey they are erected by a set of muscles which join them to the jaw. His mouth is so situated under the head that he is obliged to turn himself on one side before he can grasp any thing with those enormous jaws.
What is the main idea of this passage?
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Correct Answer: D
The correct answer is "sharks are large, powerful, and dangerous." This is the main idea because it summarizes the focus of the passage as a whole. The passage talks about the size of sharks, the power of their teeth and jaws, and the danger they pose to sailors. The other answer choices are not main ideas--they are supporting details, and/or not mentioned in this passage. For those reasons, the best choice is "sharks are large, powerful, and dangerous."