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For a topic sentence, or the beginning sentence that signifies the main idea of the paragraph, you could focus on the fact that there are a variety of things to do at the beach (perhaps the 3rd sentence). To create a hook, or an eye-catching sentence, you can use the childhood memory of learning to swim, but perhaps in more detail. In other words, recreate the scene rather than just saying it was a happy memory (i.e. I'm five years old and am amazed by how weightless I feel in the water). Once you create the hook and topic sentence, you can talk about other activities at the beach as your supporting evidence.
The comma should go after again, so the answer is D
