Passage 2Seoul, once a city for kings, can now claim to be a city for commuters. The third nation in the Orient to develop an underground rapid-transit system, Korea opened its first line in 1974. After 12 years of continuing construction, Seoul had finally completed the rest of its extensive subway system, capable of serving 5 million commuters a day.The 73-mile-long system is expected to alleviate the acute daily traffic congestion downtown. For an estimated $2.7 billion, the city has built one of the most modern subways in the world, replete with air-conditioning, high-tech ticket machines and escalators to deep stations. The subway stops, bucking an international tradition of dull concrete wall, are attractions themselves: many are lined with shopping arcades, others sponsor art exhibits and several are blasted out of granite and left in their natural state, creating an eerie, cave like effect.The subway is a long-term solution to transit problems in a city that is bursting with a large population. It is designed to encourage the growth of satellite cities along the lines that run outside the city proper. Efficiency, safety and economy are the catchwords of the Seoul Metropolitan Subway Corporation, which handled construction and now manages the four lines. But traditional concern for cleanliness adds to the popularity of this rapid mode of underground transportation.Recently 20 couples chose the subway as the site for their wedding ceremonies, proof that the system is heralding a new age of modern living for the inhabitants of the nation’s capital.
According to the passage, how long does it take Korea to complete its extensive subway system?