Hello Aditya!
yes you can totally opt for computer science engineering as the only thing judged by the competitive exams is your knowlegde in the subjects: Physics, Chemistry and Mathematics. If you know them, then you can surely do well in competitive exams and persue your career.
Yes you can do B.tech in computer science engineering with PCM as a background in 12 .It is not necessary that one should study computer in 11 -12 to do B.tech in CSE.In college teachers will teach you from the basics.May be you will face difficulty in starting but after sometime you will become comfortable.Main thing is that your mathematics should be at least intermediate level of 12.
Hope it answer your concerns.
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