One strategy you can use to determine the meanings of unknown words and phrases is to look at the context clues. Context clues might be something like a picture, words or phrases you already know, using what you know about the text already.
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The topic sentence for the paragraph above is A. Sentence 1. A topic sentence is supposed to give the general idea of a paragraph. Sentence 1 briefly describes what and where plates are in the Earth's crust. The rest of the sentences give more detail about tectonic plates.
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I believe the answer is the 1st one