请认真阅读下面短文，从短文后各题所给的A、B、C、D四个选项中，选出最佳选项。

All aboard! Let’s take a ride on the world’s highest railway—the Qinghai-Tibet Railway（青藏铁路）. Sit back and enjoy the journey. Get ready for the trip of a lifetime.

We’re leaving Qinghai Station. The scenery along the way is some of the wildest in all of China. Keep your eyes open as we race through this beautiful land. You might see a rare Tibetan antelope（藏羚羊）from your window.

Before we begin our journey, let’s learn about this amazing railway. It’s truly one of the great engineering wonders of the 21st century. When it was completed on July 1, 2006, the Tibet Railway set nine world records, including the world’s highest railway at 5,072 metres above sea level, the world’s highest tunnel（隧道）at 4,905 metres, and the longest tunnel ever built on permafrost（冻土）.

As we travel higher, the air gets thinner and thinner. Soon there will only be 50 to 60 percent of the oxygen which we are used to breathing. But don’t be afraid. Each rail car has two separate oxygen systems. One system spreads oxygen throughout the train at all times. And each passenger can also get a personal oxygen mask just like those used in planes.

Here are a few more things to think about as our train leaves the station. The designers solved three main challenges when building this great railway. First, the high altitude（海拔）made the task difficult because there was no enough oxygen. Second, much of the railway runs across permafrost, a type of ground that keeps changing as the weather warms and cools. Finally, the railway goes through some of the most sensitive ecology（敏感的生态环境）in China. Great care was necessary to make sure of the smallest possible influence on nature.

All three challenges were met, making our trip possible. All aboard! The train to Tibet is leaving in fifteen minutes.

（1）Where are the passengers?

A．

On a bus.

B．

On a train.

C．

On a ship.

D．

On a plane.

（2）How does the text show the railway is amazing in paragraph 3?

A．	By telling a story.	B．	By using a designer’s words.
C．	By introducing its world records.	D．	By comparing the present with the past.

（3）How many oxygen systems are there in each rail car?

A．

One.

B．

Two.

C．

Three.

D．

Four.

（4） Which of the following increased the difficulty of building the railway?

A．

The changing ground.

B．

The polluted air.

C．

The dangerous animals.

D．

The hot weather.