How long does the world's installed high-speed rail network cover? PASSAGE PASSAGE PASSAGE China is designing its next generation of trains that can carry passengers at a top speed of 500 kilometers (310 miles) an hour and cargo at 250km/h, with wheels that can adjust to fit different track gauges(轨距) used around the world. Under an ambitious government plan starting this year, the country is developing trains that can run on a hybrid-propulsion(混合推进) system that allows higher speeds, said Jia Limin, a professor at Beijing Jiaotong University. Using the enhanced technology and expanded network, "China will have the experience to operate high-speed rail networks in the world's most diverse geographic and climatic conditions, from deserts to alpine plateaus to rainforests", said Jia during an interview in Hong Kong. China's high-speed rail network is designed to operate in harsh winters in the northeastern provinces, where winter temperatures can plummet(暴跌) to 40 degrees Celsius below freezing, to the world's highest altitudes in the Tibet autonomous region, to deserts on its western frontiers. The domestic high-speed tracks already cover 20,000km, or 60 per cent of the world's installed network. That will expand to 30,000km by 2020 and 45,000km by 2030, said Jia, who heads the Chinese program to develop fast trains. The Chinese program began in 2004, when the Ministry of Railways called for bids to build a high-speed train that can run at 200km/h with a top speed of 350km/h. Three years later, the first of the Chinese high-speed trains, the CRH1A, rolled off the assembly line with a top speed of 250km/h. Chinese companies now have about 3,000km of high-speed rail contracts on their order books outside the country. Led by state-owned China Railway Rolling Stock, the firms have secured deals to build high-speed rail projects in Turkey, Indonesia, Thailand and Russia and are bidding for projects in Malaysia, the U.S. and South America. China wants to be the main builder for an overland rail network from Singapore in the southern tip of Southeast Asia northwards through Indochina and China, then westwards through central Asia and Europe. A. About 20,000km. B. About 30,000km. C. About 33,300km. D. About 45,500km.