Work with a separate question paper and answer sheet, and get told on the spot if you mark one row off.
As in the real exam, where the question paper and answer sheet are separate, you mark answers only on the OMR card on the right. If you mark a row different from the question you are viewing, you are warned on the spot.
The left side is the question paper and the right side is the answer sheet (OMR). You can mark answers only on the card on the right. Tapping an option on the left only highlights it and does not count for grading. If the question number you are reading differs from the row you actually marked, it counts as wrong-row marking and you are warned immediately.