Sir I am selected in SC preparatory course with rank 2550.Should I take preparatory course or not.Is IIT preparatory course difficult.

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You should go for prep course and may be you allotted civil or these branches.
You would be busy for JEE prep 2017 for one year at IITs or ISM.
Its a good option than taking drop for IIT JEE for next year but not a good option while comparing getting Core branches at NITs (ranking NITs colleges).