下列音节中缺少的是a的几声（   ）。

bà ba     yǎ yǔ    wá wa  

    People aren't walking any more---if they can figure out a way to avoid it.

    I felt superior about this matter until  the other day I took my car to mail a small parcel. The journey is a matter of 281 steps. But I used the car. And I wasn't in ay hurry, either, I had merely become one more victim of a national sickness: motorosis.

    It is an illness to which I had thought myself immune（免疫的）, for I was bred in the tradition of going to places on my own two legs. At that time, we regarded 25 miles as good day's walk and the ability to cover such a distance in ten hours as sign of strength and skill. It did not occur to us that walking was a hardship. And the effect was lasting. When I was 45 years old I raced –and beat—a teenage football player the 168 steps up the Stature of Liberty.

    Such enterprises today are regarded by many middle-aged persons as bad for the heart. But a well-known British physician, Sir Adolphe Abrhams, pointed out recently that hearts and bodies need proper…… is more likely to have illnesses than one who exercises regularly. And wlaking is an ideal form of exercise--- the most familiar and natural of all.

    It was Henry Thoreau who showed mankind the richness of going on foot. The man walking can learn the  trees, flower, insects, birds and animals, the significance of seasons, the very feel of himself as a living creature in a living world, He cannot learn in a car.

    The car is a convenient means of transport, but we have made it our way of life. Many people don't dare to approach Nature any more; to them the world they were born to enjoy is all threat. To them security is a steel river thundering on a concrete road. And much of their thinking takes place while waiting for the traffic light to turn green.

    I say that the green of forests is the mind's best light. And none but the man on foot can evaluate what is basic and everlasting.

阅读下面的文字，完成下题。

                                                                                画家画天空，背后有科学

      爱德华·蒙克的《呐喊》中，天空充满了红色的颜料，它们像一大块黏糊的东西，看起来如此沉重，似乎即将冲压下来。这夸张的色彩也许不是画家的随意泼墨，美国得克萨斯州立大学物理学家唐纳德·奥尔森在《天空和望远镜》杂志中指出，蒙克画中的红色天空，也许是一种自然现象。

      那些传世画作往往反映了真实的气候环境情况。希腊气象局科学家赫里斯托斯·泽雷弗斯所带领的科研小组也做了类似的研究。他们搜集了554幅日落油画和水彩画。这些画作分别出自181位著名画家之手，其中包括法国印象主义画家德加、奥地利画家克里姆特、荷兰画家伦勃朗、德国宫廷画师鲁本斯等。最后把他们的研究论文发表于《大气化学与物理》杂志上。该论文分析了16世纪以来554幅描绘日落景象的名画中红绿两色的使用频率，结果表明，在过去的100年中，大气中的悬浮颗粒物浓度一直呈上升趋势。他们认为，由自然原因和人为原因造成的大气中粉尘含量上升，会阻碍阳光进入大气层，导致全球暗化。

      英国伯明翰大学气象学家约翰·托宁多年来也一直在苦心研究各个时期英格兰各地的天气状况，他意外发现莫奈旅居伦敦期间的作品为他留下了宝贵的资料。莫奈创作于那时期的画作中，展现了阳光在泰晤士河上跳跃闪耀，天空呈现出奇异的色彩变化。约翰·托宁想到，这些丰富的颜色并非出于莫奈头脑中的艺术幻想，而来自于真实的伦敦生活。那漫天黄色，应是高硫含量的煤燃烧后烟尘和硫酸盐颗粒散布到空气中，成为水蒸气的凝结所致，在此条件下形成的雾霾比自然形成的雾更为持久。比较巧合的是，狄更斯在反映英国工业革命时期的小说《雾都孤儿》中也写道：“在城市边缘地带，雾是深黄色的。靠里一点儿，是棕色的。再靠里一点儿，棕色再深一些。直到商业区的中心地带，雾是赭黑色的。”

      科学家并且还去做了验证，选取莫奈1898—1900年间创作的9幅作品，结合他同一时期的信件和日记，与美国海军天文台的数据对比。令他们惊叹的是，莫奈身为艺术家，却如同科学研究者一样，一丝不苟地记录了创作日期及太阳所处的位置，绝无臆造。所以印象派大师的确是非常可信地用手中画笔将伦敦当年的绝世光影留存于世。不过印象派大师的绚丽色彩，可能来自PM₁₀之类的大气污染物，想到这点不免让人有点沮丧。

（节选自《北京青年报》2015年1月23日，入选本试卷时，有删改）

西北地区利用黄河水灌溉农业区有（　　）