why pairs is called the City of Lights?(请用英文回答,

there are 2 mistakes in ur qiestion. Paris,----not pairs.light---not lights.
the following answer taken from viki may help you:
Paris was nicknamed the "City of Light" (not City of Lights) originally because it was a vast center of education and ideas during the Age of Enlightenment. In 1828, Paris began lighting the Champs-Elysées with gas lamps. It was the first city in Europe to do so, and so earned the nickname "La Ville-Lumière" or The City of Light.