氟化物治疗牙本质过敏的作用机制是 A．堵塞牙本质小管，增加牙本质的硬度和抗酸，抗溶性

阅读理解。

      We can not live a modern life without traveling. The fastest way of traveling is by air. With a

plane one can travel in one day to places which it took a month or more to get to a hundred years

ago.

       Traveling by train is slower than by plane, but one can see the country he is traveling through.

Modern trains have comfortable seats and dinning-cars. They make even the longest journey

enjoyable.

       Some people prefer to travel by sea when possible. There are large luxury liners (客轮) or river

boats. They are not as fast as trains or planes, but traveling by sea is a very pleasant way to spend

a holiday.

       Many people like to travel by car. They can make their own timetable. They can travel hundreds

of miles a day, just as their wish. They can stop wherever they want to see something interesting or

to enjoy a good meal at a good restaurant, or to spend the night at a hotel. That is why people choose

traveling by car for pleasant trips, while they usually take trains or planes for business.

1. According to the passage, the fastest way of traveling is _______.

A. by car

B. by train

C. by plane

D. by sea

2. If we travel by car, we can _______.

A. enjoy the longest journey

B. travel only fifty miles a day

C. make our own timetable

D. travel to a very far place in a few minutes.

3. When people travel on business, they usually take _______.

A. a boat or a train

B. a car or a train

C. a plane or a car

D. a train or a plane

4. _______ have comfortable seats and dinning-cars.

A. Cars

B. Modern trains

C. Planes

D. Buses

5. If possible, people like traveling by sea because _______.

A. it is as fast as trains and planes

B. it is a pleasant way to spend a holiday

C. it is very cheap

D. it can stop wherever they like

2010年元旦过后，美国科幻大片《阿凡达》在我国上映。影片讲述地球人类试图从遥远的潘多拉星球开采矿产以解决地球的能源危机而与“纳美人”（该星球的土著人类）发生一系列的矛盾冲突。假如影片中“潘多拉”星球在宇宙中存在，那么它不可能属于

A．河外星系

B．总星系

C．银河系

D．太阳系

男性患者，45岁，双手骨间肌及大小鱼际肌肌肉萎缩1年，双手指活动不灵3个月，查体双手远端肌力4级，双手骨间肌及大小鱼际肌肌肉萎缩，以右侧为重，无感觉障碍，四肢腱反射活跃，双病理征阳性。

若肌电图显示神经源性改变，可见纤颤电位和正锐波，神经传导速度正常，考虑哪种病可能性大（）。

A.周围神经病

B.脊髓型颈椎病

C.多灶性运动神经病

D.脊肌萎缩症

E.运动神经元病

Our daily existence is divided into two phases, as distinct as day and night. We call them work and play. We work many hours a day and we allow the necessary minimum for such activities as eating and shopping. 46) The rest we spend in various activities which are known as recreations, an elegant word which disguises the fact that we usually do not even play in our hours of leisure, but spend them in various forms of passive enjoyment or entertainment.

We need to make, therefore, a hard-and-fast distinction not only between work and play but, equally, between active play and passive entertainment. 47) It is, I suppose, the decline of active play — of amateur sport — and the enormous growth of purely receptive entertainment which have given rise to a sociological interest in the problem. If the greater part of the population, instead of indulging in sport, spend their hours of leisure "viewing" television programs, there will inevitably be a decline in health and physique. In addition, we have yet to trace the mental and moral consequences of prolonged diet of sentimental or sensational spectacles on the screen. 48) There is, if we are optimistic, the possibility that the diet is too thin and unnourishing to have much permanent effect on anybody. Nine films out of ten seem to leave absolutely no impression on the mind or imagination of those who have seen them.

49) It is only when entertainment is active, participated in, practiced, that it can properly be called play, and as such it is a natural use of leisure. In that sense play stands in contrast to work, and is usually regarded as an activity that alternates with work.

Work itself is not a single concept. We say quite generally that we work in order to make a living. Some of us work physically, tilling the land, minding the machines, digging the coal; others work mentally, keeping accounts, inventing machines, teaching and preaching, managing and governing. 50) There does not seem to be any factor common to all these diverse occupations, except that they consume our time, and leave us little leisure.

50) There does not seem to be any factor common to all these diverse occupations, except that they consume our time, and leave us little leisure.

图A为某地等高线地形图，图B为此图甲、乙两点间地形剖面示意图，据此回答下列各题。

陡崖M最大落差（米）为（）

A．198

B．290

C．380

D．450