问题 单项选择题

某工人接触乙酸乙酯的状况为:300mg/m3浓度,接触1.5小时;200mg/m3浓度,接触2小时;180mg/m3浓度,接触2.5小时;不接触,2小时。该物质的时间加权平均浓度为( )mg/m3

A.1300
B.162.5
C.216.7
D.325

答案

参考答案:B

解析: 时间加权平均浓度可按下式计算,工作时间不足8小时者,仍以8小时计:
E=(CaTa+CbTb+…+CnTn)/8
式中E——8小时工作日接触有毒物质的时间加权平均浓度(单位:mg/m3);
8——1个工作日的工作时间(单位:h);
Ca,Cb,…,Cn——Ta,Tb,…,Tn时间段接触的相应浓度(单位:mg/m3);
Ta,TB,…,Tn——Ca,CB,…,Cn浓度下的相应接触持续时间(单位:h)。
本是基中E=(1.5×300+2×200+2.5×180+2×0)÷8=162.5mg/m3

阅读理解

阅读理解

阅读下面短文,从每题所给的A、B、C、D 四个选项中选出最佳选项。

     How could we tell time if there were no watches or clocks anywhere in the world?

     The sun was probably the world's first "clock",except(除了) in the far north,where the

Eskimos(爱斯基摩人) live.There,it's dark most of the winter,and light most of the summer.

But in most of the world,people have used ck.Even today,if you don't have a clock,you still

know that when the sun shines,it's day ,and when it's dark,it's night.the sun can also tell you

if it's morning,noon,or afternoon.

     People who live near the sea can tell time from the tides.In the day time,for about six hours,

the water rises higher and higher on the beach.And then it goes down and down for about six

hours.the same thing happens again at night.there are two high tides and two low tides every 24hours.

     Seaman on a ship learn how to tell time by looking at the moon and the stars.The whole sky is

their clock.

     In some places in the world the wind comes up at about the same time every day or changes

direction or stops blowing.In these places the wind can be the clock.

     A sand clcok is an even better clock.If you had fine dry sand in a glass shaped like the

one in the picture,you would have what is called an hour glass.The sand in the hourglass goes

from the top part to the bottom part in exactly an hour.When the hourglass is turned over,the

sand will take another hour to go back again.

1.According to the passage,there are _______ ways to tell time besides the clock and the watch.

A. 3      

B. 4    

C. 5    

D. 7

2.The Eskimos can't use the sun for a clock because ________.

A.they know very little about the sun      

B.the sun there never goes down in winter.

C.It's too cold for them to go out to watch the sun  

D.It has long dark winters and long light summers

3. "tides" in the passages means ________.

A. 洋流    

B. 潮汐  

C.  海啸

D. 波浪

4. In which page of a newspaper can you most probably read this passage?

A. news    

B. science    

C. business    

D. advertisement(广告)

5. What's the best title?

A.Different Ways to Tell Time

B.Useful Machine to Tell Time

C.The History of the Clock    

D.The Development of the Clock

单项选择题