Assessment of unregistered persons: authorities may assess unpaid tax by best judgment within prescribed period, ensuring right to be heard. Where a person liable to register fails to obtain registration or whose registration was cancelled but who remained liable to pay tax, the proper officer may assess that person's tax liability to the best of his judgment for the relevant tax periods and issue an assessment order within five years from the date specified for furnishing the annual return for the financial year to which the unpaid tax relates; provided that no such assessment order shall be passed without giving the person an opportunity of being heard.