怎样理解采样过程中的“时域离散化，频域周期化”。

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由模拟信号傅里叶变换的性质可知，两个信号若在时域是相乘的关系，映射到频域则为卷积的关系，时域中的连续信号经单位冲激抽样后，在频域中产生周期性函数，其周期等于抽样角频率。

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In the following passage, there are 25 blanks representing words that are missing from the context. You are to put back in each of the blanks the missing word. Write your answers on the ANSWER SHEET. The time for this section is 25 minutes.
Of all the employed workers in the United States, 12.5 million are part of a temporary workforce. The United States Bureau of Labor Statistics counts 9.2 percent of workers as those who have (1) they term "alternative employment arrangements." The government predicts that temporary staffing agencies (2) experience growth of 49 percent by 2010. If numbers increase at the rate (3) , these companies will add 1.9 million new jobs by 2010. That’s more than any (4) industry.
Temporary workers were the first ones to be affected by the recent economic recession, but they are (5) to be the first to regain positions as the economy picks up and companies work to rebuild. Employers use temporary workers as a resource because (6) offer
flexibility and come at a low cost. Many temporaries are categorized as "independent contractors" instead of employees, (7) allows the employer to avoid (8) certain taxes. Because they are not required to offer benefits to temporary workers, companies can save a great deal of money by hiring temps. Usually, temporary workers can qualify (9) benefits if they work for a staffing company for a certain length of time. Most temps, however, do not continue with one company long (10) to qualify for them because they regard the position as a step on the ladder to a (11) position later.
According to Richard Wahlquist, president of the American Staffing Association, 75 (12) of temporary workers hope for transition out of the temporary staffing category (13) a period of time. Wahlquist finds that temporary workers spend about 11 weeks on various assignments before their (14) out of the temporary workforce. Many use temporary positions to gain experience and skills (15) they move on to better jobs. Wahquist says that the temp workforce as a whole turns (16) 400 percent each year.
Tom Dilworth, research director at the Employment Policies Institute, explains (17) some workers like having temporary jobs (18) it affords them a great deal of elasticity with time and everything. Some temps only have a limited (19) of time to work and temp agencies can help coordinate jobs to fit their schedules. Other sometimes-employees use temporary jobs to get an (20) to a company from the inside, to get a foot in the door of a certain business or career. (21) others take temporary positions in hopes their employers will change them over to permanent positions eventually.
Nearly all the industries in the United States use temporary workers. (22) to Tim Costello, coordinator of the North American Alliance for Fair Employment, the growth of temporary labor threatens (23) job security of permanent workers who fear replacement,as well as the temps who are more accustomed to turnover. He predicts that there will continueto (24) a gradual shift from permanent employees to contingent staffing, and that such a (25) in workplace demographics will "lead to lower wages, poorer working conditions, and more instability."

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单项选择题

正确的健康概念应指( )。

A．没有疾病
B．身体壮实
C．不但躯体健康而且心理健康
D．心理素质高

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