气体灭火系统中容器阀又称为瓶头阀，按启动方式可分为六种类型，下列不属于六种类型中的是

预防性维修的作用有()。

A.能够降低故障率

B.将事故隐患消除在初级阶段

C.提高系统的稳定性和可靠性

D.保障房屋及设施设备的正常使用

填入横线上的句子，与上下文衔接最恰当的一项是             （   ）

自从地球印下人类的足迹以来，地球的容颜逐渐由美变丑了。特别是进入工业社会以后，人类在向大自然索取大量财富的同时，也排放大量废气、废水、废渣，污染着人类赖以生存的           。地球伤痕累累，公害频频发生。           人们感到生活在一个缺乏安全、不健康、不舒畅的环境中。

A．蓝天、长河和沃野“三废”的阴影像幽灵在世界各地游荡。

B．土壤、水源和空气“三废”的阴影像强盗在世界各地横行。

C．沃野、长河和蓝天“三废”的阴影像强盗在世界各地横行。

D．空气、水源和土壤“三废”的阴影像幽灵在世界各地游荡。

阅读理解。

                                                        The unknown world's mystery

      The sea is the biggest unknown part of our world.  It covers seventy-one percent of the earth.  There

is still much to be discovered about this vast blanket of water.  If the waters of the ocean could be removed,the sea floor with its wide

valleys,irregular mountains and rivers in the sea would be an unbelievable sight.

     Tourism under the deep blue sea

     Tourists can now explore the world beneath the waves without ever getting wet. From tourist

submarines to underwater dining,undersea tourism is making a big splash.

     Tourist submarines (潜艇)

     The popularity of submarine tours has grown rapidly in the last decade.  Now,tourist submarines

operate in more than 20 locations worldwide,including the waters around Okinawa and Taiwan's Green

Island. The submarines carry nearly 2 million tourists each year. Large windows offer passengers

spectacular views as the submarines explore colorful coral reefs and sunken ships. Most one-hour tours

cost about $80.

      Underwater dining

       For a more upscale experience,tourists can enjoy fine .dining under the sea.  Last summer the Hilton Maldives Resort opened the world's first underwater restaurant,Ithaa. Diners walk down a staircase into

the restaurant,which rests

35 feet beneath the waves of the Indian Ocean. Transparent walls surround the dining area,and you can

treat guests to a panoramic(全景) ocean view. But this excellent view doesn't come cheap. The tasting

menu costs $200 per person.Be sure to make reservations early,too. The restaurant seats only 14 people.

1. What does the author want to tell us in Paragraph 1?      

A. The sea is the biggest unknown part of our world.

B. The sea covers seventy-one percent of the earth.

C. Some information about the sea floor.

D. The reasons for researching undersea tourism.

2. What does the author really mean by saying "undersea tourism is making a big splash" in Paragraph 2?   

A. Undersea tourism is a new fashion in travelling.

B. Your clothes will be made wet by the water splash.

C. Undersea tourism is dangerous.

D. There is still a long way for undersea tourism to go.

3.  Which of the following statements about tourist submarines is RIGHT?   

A. Tourist submarines only have a history of about 10 years.

B. There are tourist submarines in every part of the world now.

C. Tourist submarines have carried about 2 million tourists so far.

D. People can enjoy tourist submarines in the waters around Okinawa.

4.  The walls of the restaurant Ithaa are most probably made of ______ .  

A. stone        

B. brick          

C. glass          

D. steel

5.  From the passage,we can infer that the author _____.

A.  doesn't think underwater tourism is a good idea  

B.  shows great interest in underwater tourism

C. is worried about underwater tourism        

D. is probably a scientist who studies

硅酸盐水泥的适用范围是（）。

A．受海水侵蚀的工程

B．受化学侵蚀的工程

C．需快硬早强的工程

D．大体积混凝土工程

筒式反应器每段床层外循环的物料经冷却器后，进入的位置是该段的（）。

A、顶部

B、底部

C、中部

D、任一位置