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【单选题】
设 S 1 =1+ 1 1 2 + 1 2 2 ， S2=1+ 1 2 2 + 1 3 2 ， S3=1+ 1 3 2 + 1 4 2 …， S n =1+ 1 n 2 + 1 (n+1) 2 ，设 S= S 1 + S 2 +…+ S n ，其中n为正整数，则用含n的代数式表示S为（ ）A.
n 2 -n-1 n+1
B.
n 2 +2n n+1
C.
1 n(n+1)
D.
2n+1 n(n+1)
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