读一读，回答问题。

Li Ming: Excuse me! Where is the library?

Miss Liu: Across from the office.

Li Ming: Is that far from here?

Miss Liu: No! It's just up the stairs! I can show you.

Li Ming: Thank you!

Miss Liu: Follow me! Here it is!

Li Ming: Thanks!

Questions:

1. Is the library far from the office?

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2. Does Miss Liu help Li Ming get to the library?

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已知S_n是等差数列{a_n}的前n项和，S₁₀＞0并且S₁₁=0，若S_n≤S_k对n∈N^*恒成立，则正整数k构成集合为（　　）

A．{5}

B．{6}

C．{5，6}

D．{7}