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os.path模块中的______方法用来测试指定的路径是否为文件。

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os模块的______方法用来返回包含指定文件夹中所有文件和子文件夹的列表。

对于设定提存计划，企业应当根据在资产负债表日为换取职工的会计期间提供的服务而应向单位主体缴存的提存金，确认为应付职工薪酬。（）

A. 对
B. 错

1. Why is a data stream usually represented as an aperiodic digital signal? Why is amodulated signal represented as an aperiodic analog signal?1．为什么一个数据流通常表示成一个非周期性的数字信号？为什么一个调制信号表示成一个非周期性的模拟信号？Answer:A data stream is of digital signal, but not periodic. Thus, it is represented as an aperiodic digital signal, see Figure 2.2 (b). After a digital signal modulates carriers, which is of periodic analog signals, a resulted signal forms. This resulted signal is called a modulated signal that has the same type of carriers. Therefore, a modulated signal is usually represented as an aperiodic analog signal, see Figure 2.1 (b).

2.Compare the number of required frequencies and the size of bandwidth to represent the following signals: (a) periodic analog, (b) aperiodic analog, (c) periodic digital, and (d) aperiodic digital.2．比较需要的频率数量和带宽大小来表示下列信号：（a）周期性模拟(b)非周期性模拟，（c）周期性数字，及（d）非周期性数字。Answer:Period analog signal: one discrete frequencyAperiod analog signal: bandwidth-limited continuous spectrum Period digital signal: infinite discrete frequenciesAperiod digital signal: continuous spectrum , but bandwidth is unlimited.