Spoiler alert: The answer is below the line.
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The right answer is H. The antecedent of the pronoun they is the noun phrase gravel and dolomite prairies. Therefore, Sentence 4 should come right after a sentence that makes this apparent. It should also not come between Sentences 3 and 5, as Sentence 5 is clearly supposed to immediately follow Sentence 3. And it makes sense that Sentence 4 should be next to Sentence 2, as both concern the physical characteristics of gravel and dolomite prairies. Of the choices, only H (which places Sentence 4 between Sentences 2 and 3) resolves all of these issues.
The right answer is NOT F because Sentence 5 needs to come immediately after Sentence 3.
The right answer is NOT G because this would open a paragraph with a pronoun before making it clear what the noun it represents is.
The right answer is NOT J because the beginning of Sentence 6 refers to the end of Sentence 5, and so nothing should come between them.