电力机车总风缸最低安全工作风压不能低于（）。 A.350kPa B.400kPa C
题型：单项选择题
问题：
电力机车总风缸最低安全工作风压不能低于（）。
A.350kPa
B.400kPa
C.450kPa
D.500kPa
题型：单项选择题
The Grand Canyon (峡谷) in Arizona extends four hundred fifty kilometers. The canyons look as if they formed as the earth split apart. They were actually cut by rivers.
Looking at the Grand Canyon is like looking back in time. Forty million years ago, the Colorado River began cutting through the area. At the same time, the surrounding land was pushed up by forces deep within the Earth. Rain, snow, ice, wind, and plant roots rubbed away at the top of the new canyon. Below, the flowing river continued to uncover more and more levels of ancient rock. Their appearance changes endlessly -- with the light, the time of year, and the weather.
The Grand Canyon has several weather environments. The top is often much different from the bottom. On some winter days, for example, you may find cold winds and snow at the top. But at the bottom, you may find warm winds and flowers.
The Grand Canyon is known as one of the seven wonders of the natural world. About five million people visit the canyon each year. Most visitors walk along paths part way down into the canyon. Some visitors ride mules to the bottom and back.
There are several other ways to visit the Grand Canyon. Hundreds of thousands of people see the canyon by air each year. They pay a helicopter or airplane pilot to fly them above and around the canyon.
About twenty thousand people a year see the Grand Canyon from the Colorado River itself. They ride boats over the rapid, rocky water. These trips last from one week to three weeks.
Visitors can see the Grand Canyon in still another way. A huge glass walkway, called the Skywalk, extends from the edge and is hanging above the bottom of the canyon.
Some people say the Skywalk is an engineering wonder. However, other people have criticized the Skywalk and future development. They say it harms a national treasure and reduces the enjoyment of nature in the Grand Canyon.
1. We can learn from the first paragraph ____________.
A. the Grand Canyon is the most beautiful place in America
B. the Grand Canyon is four hundred fifty kilometers long
C. the Grand Canyon used to be a dried river
D. the Grand Canyon used to be a deep opening in the earth
2. According to the passage, people can visit the Grand Canyon in ______ ways.
A. 3 B. 4 C. 5 D. 6
3. We can learn that the author of the passage hold a(n) ______ attitude towards the Skywalk.
A. positive B. negative C. objective D. critical
4. All the following are true about the Grand Canyon EXCEPT ____________.
A. The top of the Grand Canyon is always cold.
B. The Grand Canyon displays different colors in different times.
C. The Grand Canyon is as old as forty million years old.
D. Every year more people see the Grand Canyon by air than by boat.