设等差数列的前项和为，若，则必定有( ) A. ,且 B. ,且

设等差数列的前项和为，若，则必定有( )

A. ,且 B. ,且 C. ,且 D. ,且

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设等差数列的前项和为，若，则必定有( ) A. ,且 B. ,且

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