根的生长主要原因：①（）细胞数量的增加、②伸长区细胞体积的增大。

Many of the most damaging types of weather begin quickly, strike suddenly, and disappear rapidly, destroying small areas while leaving neighboring areas untouched．Such event as a tornado struck the northeastern part of Edmonton, Alberta, in July 1987．Total damages from the tornado went beyond $250 million, the highest ever for any Canadian storm．

Traditional computer models of the atmosphere have limited value in predicting short-lived local storms like the Edmonton tornado, because the available weather data are generally not detailed enough to allow computers to study carefully the slight atmospheric changes that come before these storms．In most nations, for example, weather-balloon observations are taken just once every twelve hours at places separated by hundreds of miles．With such limited data, traditional forecasting models do a much better job predicting general weather conditions over large areas than they do forecasting specific local events．

Until recently, the observation intensive method needed for exact, very short-range forecasts, or “Nowcasts,” was not possible．The cost of equipping and operating many thousands of conventional weather stations was extremely high, and the difficulties concerned in rapidly collecting and processing the weather data from such a network were hard to overcome．

Fortunately, scientific and technological advances have overcome most of these problems．Radar systems and satellites are all able to make detailed, nearly continuous observation over large areas at a lower cost．Communications satellites can send out data around the world cheaply and immediately, and modern computers can quickly collect and analyze this large amount of weather information．

Meteorologists（气象学者）and computer scientists now work together to design computer programs and video equipment able to change weather data into words and graphic displays that forecasters can understand easily and quickly．As meteorologists have begun using these new technologies in weather forecasting offices, Nowcasting is becoming a reality．

1．Why can’t traditional computer models predict short-lived local storms?

A．The weather data people collect are often wrong．

B．Detailed weather data in some small areas are not available．

C．The computers are not advanced enough to predict them．D．The computers are not used to forecast specific local events．

2．The word “Nowcast” in Paragraph 3 means ________．

A．a network to collect storm data     

B．a way of collecting weather data

C．a more advanced system of weather observation

D．a forecast which can predict weather in the small area

3．What can make “Nowcasts” a reality according to the passage?

A．Scientific and technological advances． B．Advanced computer programs．

C．Computer scientists．              D．Meteorologists．

4．What does the passage mainly talk about?

A．The advantages of “Nowcasts”．        B．A tornado in Edmonton, Alberta．

C．The difficulty in predicting tornado．  D．A great development in weather forecast．

牙颈及牙根表面的淡黄色组织为

A．牙骨质
B．牙釉质
C．牙本质
D．牙髓
E．髓腔

THDS对货车滚动轴承的热轴故障报警，具体标准由（）和红外线设备生产厂家共同确定，并报铁道部车辆主管部门备案。

A.铁路局

B.铁道部

C.车辆段

D.调度所

退休后的收入实际上是退休后的生活保障，不同退休养老规划工具的收入构成了保障退休养老金的不同“防线”。其中，保障退休后生活的第四道“防线”应当是( )。

A．国家的社会保险制度

B．企业年金收入

C．商业性养老保险

D．个人储备的退休养老基金

突变和基因重组产生了生物进化的原始材料，现代生物技术也是利用这一点来人为地改变生物遗传性状，以达到人们所期望的目的。下列有关叙述错误的是（）

A.转基因技术造成的变异，实质上相当于人为的基因重组，定向改变了生物性状

B.转基因技术可以在不同物种之间进行，它突破了物种间生殖隔离的限制

C.人工诱变没有改变突变的本质，但却因突变率的提高而实现了定向变异

D.经过现代生物技术的改造和人工选择的作用，许多生物变得更适合人的需要