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Please reading the following passage then answer the question below.
The sentences are numbered to help you answer the question.
(1) The solar system is made up of the sun and the planets that go around it. (2) The planet closest to the sun is called Mercury. (3) The planet closest to the sun is Venus. (4) Earth is the third planet and Mars is the fourth. (5) Life on Earth is thought to have begun about three billion years ago. (6) For many millions of years, the most common lifeform was bacteria. (7) Scientists have tried to understand how plants and animals developed from these simple, one-celled creatures.
The topic sentence in the first paragraph is number
A. 4
B. 1
C. 2
D. 5