Welcome on the FORUM, Mr. Maaz.
I would like to add in the above-stated words of the respected member that you should not be worried about the optional subjects selection. You should start reading Dawn newspaper on daily basis if you are not doing so. Secondly, pick a compulsory paper and read one or two recommended books. Do this in one month. You will feel easy and comfortable to go with the schedule of CSS, and during this one month, try to read the syllabus and past papers of the subjects that can be studied at the best possible manner by you. Your subject selection will not hinder or matter in the allocation of your desired Group.

In the end, I would like to say you will find everything related to your educational endeavors at this prestigious Forum. All powers to your, dear!

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"But screw your courage to the sticking place,
And we'll not fail." _Shakespeare, 'Macbeth')