题目信息
Since around 1970, with world population of 3.5 billion, they have doubled it to our current level of over 7 billion in 2012.
A:with world population of 3.5 billion, they have doubled it
B:with a world population of 3.5 billion, doubling it
C:when the world's population was 3.5 billion, doubling it
D:when the world's population was 3.5 billion, it has doubled
E:when the world's population were 3.5 billion, they have doubled it
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正确答案：
D:when the world's population was 3.5 billion, it has doubled
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