I want help of indian frnds.?

how is ias merit is prepared in the pre examination,as we know that there are many optional subject each has to pick one of them.is merit prepared on the basis of higher marks regardless of subject or subject-wise,for example if there are A B C D E are five category and each has BOTANY ZOOLOGY POLITICS HISTORY AND MATH and A category of botany obtain 235 245 255 265 AND 275 marks out of three hundred ,will upsc select the student of only botany category becoz of higher marks or from each subject with equal proportion?