The bicycle is one of the simplest yet most useful inventions in the world. What is the most surprising is that it was not     1     earlier, although the great inventor Leonardo da Vinci had     2     pictures for bicycles and flying machines and some other things. Those things were not produced     3     long after he died.

A person riding a bicycle uses very little     4     to make the bicycle move, and there is no pollution at all when you are riding. Even so, in most developed     5    , most people don’t go to work by bicycle. It is not     6     the bicycles are expensive or people feel     7     if they ride to work. It’s because the number of cars on the roads becomes larger. It certainly becomes more     8     to ride a bicycle. As a result, more people put their bicycles away and go to work     9     their cars. And in this way, the situation is made more serious. Perhaps the best way to make riding safer and more popular is to     10    , paths only for bicycles.

Imagine a classroom missing the one thing that has long been considered a(n)     1     part to reading and writing? Paper. No notebooks, no textbooks, no test paper. Nor are there any pencils or pens which always seem to run out of ink when we really need it.

A “paperless classroom” is what more and more schools are trying to     2    .

Students don’t do any handwriting in this class. Instead, they use specially-designed, or palm size computers. The teacher downloads texts from Internet     3     and sends them to every student’s personal computer.

Having computers also means that students can use the Web. They can     4     information on any subject they’re studying: from maths to social science.

Exams can go online too. At a high school in Tennessee, US, students take tests on their own computer. The teacher records the     5     on the network for everyone to see and then copies them to his own electronic notebook.

A paperless classroom is a big step     6     reducing the waste of paper. High school teacher Stephanie Sorrell in Kentucky, US, said she used to give about 900 pieces of paper each week to each student. “Think about the money and trees we could save with the computers.” She said.

But, with all this technology, there’s always the     7     that the machines will break down. So, to avoid (回避) it, paper textbooks continue to be     8     available for these hi-tech (高科技) students.

On May 17, China launched a new satellite (卫星) at the Xichang Satellite Launch Center. It was the forty-fifth Beidou satellite that China had sent into space.

So what is Beidou? Beidou is a positioning system (定位系统). It     1     by China. China is the third country in the world to have its own positioning system. By     2     Beidou, China no longer has to depend on foreign systems. This is very important for national safety.

Work on Beidou     3     in 1994. However, the first two Beidou satellites were not sent into space until 2000. Since then, China has launched dozens of Beidou satellites. These satellites have formed a global network. Now, China is further improving the system. It plans     4     up ten more Beidou satellites. If everything     5     well, Beidou will be completed before 2020.

The completed Beidou     6     more powerful than any other positioning system. Yang Changfeng is a designer of Beidou. He says the system     7     the sway (摇摆) of a building in strong winds.

So far, Beidou     8     much benefit (福利) to our lives. In many cities, thousands of buses have been using Beidou. While we     9     for a bus, we can know when it is about to come.

Many smartphones (智能手机) have also been using Beidou. Many people say, “Let me     10     on GPS.” “In fact, their phones are using Beidou.” jokes a salesperson in Beijing.