设在图3-8和图3-9所示的系统中，R1，R2，R3为3个加工部件，每个加工部件的失效率均为丸，可靠性均为只。则图3-8系统的失效率为(20)，可靠性为(21)。图3-9中系统的失效率为(22)，可靠性为(23)。若每个加工部件的平均无故障时间为5000小时，则图3-9中系统的平均无故障时间为(24)小时。

A．λ/3
B．λ3
C．3λ
D．1-λ3

Where did the waltz originate？（）

A．Germany.
B．Austria.
C．Vienna
D．Switzerland.

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W: You seem very tired lately.M: It’s my new job. It’s hard work, and the hours are long.Q: Why is the man tired?（）

A．His job is difficult.
B．His job is not interesting.
C．He doesn't know how to do his job.
D．He only sleeps for a few hours a night.

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