Final answer:
The basis for topic sentences in a research essay is usually the subjects or topic areas into which the data has been grouped.
Step-by-step explanation:
When outlining a research essay, the basis for the topic sentences can vary depending on the approach taken. Typically, the topic sentences are based on the subjects or topic areas into which the gathered data or research has been grouped. These topic sentences serve as a guide to the main points or arguments that will be discussed in each paragraph of the essay.
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