i m getting mechanical at iitkgp ,chemical at any of the iits, civil at any and aerospace at any. in future i want to go for ias. also i want to be in some big city. which one should i choose?

Go for IIT Kharag pur your aspirations will full fill

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IAS has not much to do with course in IIT. So, Mechanical at Kharagpur will be better. Staying in big city has nothing to do with course or branch in IIT. It is entirely personal choice or moreover depends on your rank in IAS entrance.