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Some like Mutyala Raju had cracked it..i have friends who're into IES and have single digit rank in GATE though they have these optionals in civils they personally suggest never take it ...
non engineering subjects like public administration, geography, psychology, sociology are better...unless u r very sure u r in the top in ur engineering discipline...it is a bit difficult considering the vastness of syllabus and the chances of getting through scaling of
UPSC considering the number of people who take it...
everyone has his own inclinations so better take self decision after seeing Syllabus and Q paper of previous years..
may be u might also want to have a look at what are the average scores in mains of the optional u r planning ..
but interest is the only thing that must be the final criterion (though u have to spend time to look at other subject to see if it interests you more
)...
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hope it helps and all d best