I was one of the people     1     work on Qinghai­Tibet Railway，which was completed by     2     (overcome) the most difficult engineering challenges to protect the delicate ecosystem.To allow animals to move     3     (safe) and freely，thirty­three passages have been built.To prevent damage to wetlands and grasslands,675 bridges，which is about 160 kilometres in     4     (long)，were build and 140,000 square metres of wetland     5     (move) to protect its distinct ecosystem.Tanggula Station，    6     (locate) at over 5,000 metres above sea level，is the highest railway station in the world.To keep us     7     (health)，several oxygen­making stations were constructed.To protect Cuona Lake     8     construction waste，a twenty­kilometre wall was built.Thanks to our     9     (effort)，passengers who have been attracted by the Qinghai­Tibet Plateau，have been enjoying these     10     (magic) landscapes.

The London Underground is the oldest underground network in the world. It normally     1    (operate) between 5 am and midnight. Some lines stay open throughout the night on special     2    (occasion), like New Year’s Eve. The London Underground stations can get very     3    (crowd) during the weekday rush hours. Over the decades the London Underground stations have been modernized. In the past few years, many     4    (equip) with Wi-Fi access to make passengers’ journeys as comfortable as possible.     5     well-known symbol of the London Underground, a red circle with a blue bar, was created at the     6    (begin) of the 20th century and has not changed much since then. Some of the London Underground stations are buildings     7     have special architectural values. Many original stations have been restored     8     they look similar to the way they did over a century ago.

With so many people     9    (use) it every day, the London Underground faces environmental problems. Because the water level of the Thames is on the rise     10    (constant), thousands of cubic meters of water must be pumped out of the underground stations every day.

A new form of transport will be with us in the next few years — flying taxis. Many of us grew up     1     (watch) sci-fi movies with flying cars. Science fiction is now becoming science fact. The Japanese airline ANA has teamed     2     with ＆ U. S. tech start-up called Joby Aviation. The two companies aim     3     (start) operating air taxis at the 2025 World Expo in Osaka. They are     4     (current) working together on building the flying vehicle. They also need to work out     5     traffic rules should be followed by the taxis. The five-seat, all-electric taxi will be able to take off and land vertically (垂直地). It will have a flight range of 241 kilometers     6     a top speed of 321 kph.

Joby’s CEO said the taxis would be good for the environment. He told reporters, “Joby exists to help people save time while reducing their carbon footprint. Japan offers us     7     opportunity to do just that with 92 percent of the population living in urban areas.” The president of ANA, Koji Shibata, was also     8     (excite) about the project. He said, “ANA has 70 years of experience delivering safe flights to     9     (customer) across the world. Being able to provide them with the option to travel rapidly, and sustainably, from an international airport to a downtown location     10     (be) very appealing.”

The abacus (算盘) was a great invention in ancient China. The abacus     1     (invent) on the basis of small rods (棒). Later, with the     2     (develop) of productivity, the amount of calculation was     3     (great). And calculation with counting rods limited the calculation. Thus, people invented a more     4     (advance) counter—the abacus.

With the application of the abacus, people summarized many abacus rhymes (珠算口诀),     5     (increase) the calculating speed. By the time of the Ming Dynasty, people could use the abacus in addition, subtraction, multiplication and division,     6     were widely used in calculating weight, amount, space and volume.

Since it was easy to make or buy     7     abacus. And it was also easy to remember abacus rhymes. The abacus was widely used in ancient China. Later, the abacus was     8     (gradual) spread into Japan, Korea, America and some countries and regions in Southeast Asia.

People find that using abacuses can improve thinking and practical abilities in addition     9     providing convenient calculation. Since it     10     (require) cooperation of mind, eye and hand, it is a good way to improve the comprehensive reaction ability.