Hi Swetha, Welcome to English Forums. The two prepositions 'in' and 'at' have many uses, but when we are speaking of a physical location: we use 'in' when we are considering the two- or three-dimensional quality of the location-- 'I live in Chennai' -- you are thinking of your home, a building located somewhere within the city boundary. 'At' is used when the location is being thought of as a non-dimensional point, as on a map, or on the Earth-- 'my plane landed at Chennai before continuing to Mumbai'.
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