It is said that by( )the population of the world may be seven bil_英语解答

It is said that by( )the population of the world may be seven billion.
A.2010 year B.2010 the year C.year 2010 D.the year 2010
我知道正确答案是什么,关键这道题考察的语法是什么?

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解答

选D
by the year ---
表示到某年为止