What’s the topic sentence in the first paragraph?

(3)So, Mercury is the closest planet to the Sun. And then Venus is the second closest planet to the Sun, and Earth is the third closest planet to the Sun. The distance of Mercury to the Sun is actually an average. Mercury follows an elliptical orbit around the Sun..

(4)When you read "full sun," it means that a plant needs direct, unfiltered sunlight for at least 6 hours a day. This is the minimum amount of light a sun-loving plant needs to thrive. As you have noticed, the intensity of sunlight varies depending on the time of day.

(5)The age of the Earth is about 4.54 billion years; the earliest undisputed evidence of life on Earth dates from at least 3.5 billion years ago. There is evidence that life began much earlier.

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the answer is a.

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