CSS is exam of critical and critical ability rather than the power of knowledge.
We are free in our choice but as a science student we feel more pressure as we do not know the ABC of many arts subjects.
I.R is best optional after revised syllabus.
Sociology and Public Administration can be prepared without prior knowledge in short time.
I done master in Pure Math but my optional
I.R
Public Administration
Psychology
Muslim Law
Islamic History & Culture.
I opt Islamic Subjects as I am interested in these topics.
I.R is remaining option because science subjects need more and more time and also need a lot of practice.
Computer Science in good if you have firm grip as you did master in this subject.
Go through syllabus and past papers.
Best of Luck

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