 Complete the following phrase:  Paris is a major railway, highway, and air-transport hub served by two international airports: ParisCharles de Gaulle the second-busiest airport in Europe and ParisOrly. Opened in 1900, the citys subway system, the Paris Métro, serves 5.23 million passengers daily; it is the second-busiest metro system in Europe after the Moscow Metro. Gare du Nord is the 24th-busiest railway station in the world, but the busiest located outside Japan, with 262 million passengers in 2015. Paris is especially known for its museums and architectural landmarks: the Louvre was the most visited art museum in the world in 2019, with 9.6 million visitors, and remained the most-visited art museum in 2020 when the number of visitors dropped to 2.7 million due to the COVID-19 pandemic. The Musée dOrsay, Musée Marmottan Monet and Musée de lOrangerie are noted for their collections of French Impressionist art. The Pompidou Centre Musée National dArt Moderne has the largest collection of modern and contemporary art in Europe. The Musée Rodin and Musée Picasso exhibit the works of two noted Parisians. The historical district along the Seine in the city centre is classified as a UNESCO World Heritage Site; popular landmarks there include the Cathedral of Notre Dame de Paris on the Île de la Cité, now closed for renovation after the 