下面哪种访问控制模式需要对客体进行定义和分类？（）A、基于角色的访问控制 B、自主访问

一名同学为了研究某种琴弦的发声频率与琴弦长度L及弦内张力T的关系，设计了如图所示的实验：将琴弦一端固定，另一端通过悬挂不同个数的钩码来改变琴弦的张力T，通过改变支撑木条的间距来改变琴弦发声的有效长度L．在只让钩码质量m或只让琴弦有效长度L变化时，测出琴弦振动频率f，并绘出了如下图所示的两个图象．

（1）该同学研究琴弦发声频率与琴弦长度及弦内张力的关系时使用的方法是______．

（2）由图象可知琴弦调节的越紧，所发出的声音频率______（选填“越高”、“不变”、“越低”）．

等差数列{a_n}的前n项和为S_n，已知S₁₀＝0，S₁₅＝25，则nS_n的最小值为________．

— What a nice MP5! Whose is it? 

— It's           . My father bought me last week. [ ]

A. me

B. him

C. his

D. mine

北京是中国的首都。

Beijing is the                    China.

Packet-switching wireless networks are preferable (71) when transmissions are (72) because of the way charges are (73) per packet. Circuit-switched networks are preferable for transferring large files or for other lengthy transmissions because customers are (74) for the (75) of time they use the network.

A．charged

B．fined

C．free

D．controlled