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问题 单项选择题

有一水库水源水含有沉速为0.2mm/s、0.3mm/s、0.4mm/s、0.5mm/s的颗粒占所有颗粒的质量比例为95%。经试验测得沉速≥0.4mm/s的颗粒沉淀1.5h去除的质量占所有颗粒的质量比例为65%。按此试验设计沉淀面积为1450m2的平流式沉淀池。当处理水量为50000m3/d，沉淀时间1.5h，进水悬浮物固体含量为8mg/L，出水悬浮物固体含量为1mg/L时，推算沉速为0.3mm/s的颗粒占所有颗粒的质量比是（）。

A．20%

B．23．75%

C．25%

D．30%

答案

参考答案：A

解析：

本题已知条件不太严谨，有5%的颗粒没明确其沉速分布，导致答案不唯一。

解1：据题意，认为有5%的颗粒沉速小于0.2mm/s，不妨假定其平均沉速为0.1mm/s。

P0.4+P0.5=65%

P0.2+P0.3=(95-65)%=30%

设0.3mm/s的颗粒占总颗粒量的百分率为x即P0.3=x，写出去除率计算式：

解2：据题意，也可假定有5%的颗粒沉速大于0.5mm/s，那么

设0.3mm/s的颗粒占百分率为x即P0.3=x

计算题

1232﹣122×124
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单项选择题

Passage 1: The Blacks had never been rich enough to spend their summer holidays in a luxurious place. This year, however, Mr. Black made a lot of money and his wife said that she would like to go to Rome for a change. Mr. Black thought it’s a good idea. He said they should stay at a really good hotel while they went around the famous city.They flew to Rome, and arrived at their hotel late one evening. They thought that they would have to go to bed hungrily because no meals were served in cheap hotels where they used to stay. They were therefore surprised when the waiter asked whether they would be having dinner there at night.“Are you still serving dinner now” asked Mrs. Black. “Certainly, madam.” answered the waiter. “We serve it until half past nine.”“What are the time for meals then” asked Mrs. Black.“We serve breakfast from seven to half past eleven in the morning, lunch from twelve to three, tea from four to five and dinner from six to half past nine.”“But that hardly leaves any time for us to see the sights of Rome!” said Mrs. Black in a disappointed voice.

Mrs. Black wished ________.

A．to go to Rome to change the money

B． to go to Rome for a visit

C．to go to Rome to exchange the money

D．to go to Rome for a party
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