Tianwen I, the unmanned probe for China’s first independent Mars mission,【小题1】(land) on the Red Planet on May 2021, marking an important step in China’s exploration of the outer space. China has become the second country to land on Mars. The landing【小题2】(see) now by the world as another milestone in the development of China’s aerospace(航空航天) industry. And China’s space station core module Tianhe entered pre-set orbit in April,【小题3】marked a solid step in building the country’s first space station.

By far, the most 【小题1】(technology) challenging railway ever built was probably the Qinghai-Tibet Railway, completed in 2006, which connects Xining to Lhasa. The railway was first planned in the 1950s, but the technical difficulties were too much 【小题2】 (overcome). It was not until 1984【小题3】the first 814-kilometer section of the railway, from Xining to Golmud, was completed.

The biggest issue was the permafrost(冻结带)-frozen earth. In the summer, usually melting at a fast speed, the top layer of the permafrost turns to mud,【小题4】(make) the ground unstable. A【小题5】(various) of solutions were used to get around this. In some cases, railway bridges 【小题6】(build). In other cases, engineers installed pipes under the ground to keep it frozen during the summer. Finally, in some areas, it was impossible to build the railway over the permafrost, 【小题7】engineers tunneled through it.

It is hoped that the experience of building the Qinghai-Tibet Railway can be used in【小题8】 new project, linking China with Pakistan. This railway【小题9】(travel)1,000 kilometers through the Pamir Mountains, which are nicknamed the Roof of the World. Building such a railway will be【小题10】(doubt) a technical challenge. It has not yet been announced when construction on this project will start.

Renewable methods for 【小题1】 (produce) electricity, such as wind turbines and solar panels, can only provide power on the days 【小题2】 wind blows or the sun shines. Electric batteries 【小题3】 (use) to store energy are expensive and can also be polluting. Finnish scientists came up with a simple solution that puts unwanted green electricity to work, heating 100 tons of sand to around 500℃. The sand stays hot for months; when energy 【小题4】 (require), air can be pumped through it and heated up. The air in turn heats a system that supplies hot water to heat nearby houses.