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问题 选择题

（6分）波速均为v=2m/s的甲、乙两列简谐横波都沿x轴正方向传播，某时刻波的图象分别如图所示，其中P、Q处的质点均处于波峰。关于这两列波，下列说法正确的是（  ）

A．甲波中的P处质点比M处质点先回平衡位置

B．从图示的时刻开始，经过1.0s，P质点通过的位移为2m

C．从图示的时刻开始，P处质点比Q处质点先回平衡位置

D．此时刻，M点的运动方向沿ｘ轴正方向E．如果这两列波相遇不可能产生稳定的干涉图样

答案

ACE

题目分析：根据走坡法可知甲图中M点正向上振动，需要大于二分之一时间才能回到平衡位置，P点正通过最大振幅向下振动，需要四分之一周期回到平衡位置，故P处的质点比M处质点先回平衡位置，A正确；

由图知，，由解得,故P处的质点经过1s振动了二分之一周期，振动到了波谷位置，即位移为，B错误；PQ处的质点要想回到平衡位置，最小需要四分之一周期，即P处的质点需要0.5s，Q处的质点需要1s，故从图示的时刻开始，P处质点比Q处质点先回平衡位置，C正确；机械波在传播过程中，各个质点不随波传播，只在平衡位置附近振动，所以D错误；两列波相遇形成稳定的干涉图样的条件是两列波的频率相同，即周期相同，故如果这两列波相遇不可能产生稳定的干涉图样，E正确；

单项选择题

"What does the middleman do but add to the price of goods in the shops " Such remarks are aimed at the intermediate operations between manufacturers and final customers. This practice usually attracts a lot of attention from the public and the press and the operation most talked about is what is often called wholesaling.The wholesaler buys goods in large quantities from the manufacturers and sells them in smaller parcels to retailers, and for this service his selling price to the retailer is raised several percent higher. But his job is made more difficult by retail demand not necessarily running level with manufacturers’’ production. Because he adjusts or regulates the flow of goods by holding stock until required, he frees the manufacturer, to some extent, from the effect on production of changing demand and having to bear the whole risk.The manufacturer can then keep up a steady production flow, and the retailer has no need to hold heavy stocks, who can call on the wholesaler for supplies any time. This wholesale function is like that of a valve in a water pipe. The middleman also bears part of the risk that would otherwise fall on the manufacturer and also the retailer.The wholesaler provides a purely commercial service, for which he is too well rewarded. But the point that is missed by many people is that the wholesaler is not just someone adding to the cost of goods. It is true one could eliminate the wholesaler but one would still be left with his function: that of making sure that goods find their way to the people who want them.

The author quite possibly believes that the function of the wholesaler is________.

A．good but too costly

B．necessary but harmful

C．removable but necessary

D．acceptable but unnecessary
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单项选择题

患者缺失，为基牙。含孤立基牙的义齿装盒，通常采用的装盒方法是（）

A．分装法或整装法

B．整装法

C．混装法或整装法

D．分装法或混装法

E．整装法或混装法或分装法
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