Hi Murali,
In the CBSE Improvement examination a candidate can choose to give the examination in either any or all the subjects as Joint Entrance Examination demands that improvement exam must be given in all the subjects by the candidate for calculation of the aggregate. So a candidate appearing for improvement examination of CBSE will only appear for the theory papers and not the practical examination as the practical exam's marks would be carried forward to the result year of the improvement exam and thus the previous year's practical marks would only be considered.
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