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【单选题】
已知数列{a n }是等差数列，a 1 +a 3 +a 5 =105，a 2 +a 4 +a 6 =99，{a n }的前n项和为S n ，则使得S n 达到最大的n是（ ）A.
18
B.
19
C.
20
D.
21
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【单选题】The main idea of the text is that ______.
A.
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B.
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C.
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D.
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A.
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B.
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C.
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