问题描述：
The surface arca of the earth is about( )square kilometers.
A.510,000,000 B.51,000,000 C.5,100,000 D.51,000
知识小百科,请选择正确案,并翻译.
A.510,000,000 B.51,000,000 C.5,100,000 D.51,000
知识小百科,请选择正确案,并翻译.
问题解答：
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