(b)after the commencement of its business, in connection with the extension of its undertaking or in connection with its setting up a new unit,the assessee shall be allowed a deduction of an amount equal to one-fifth of such expenditure for each of the five successive tax years beginning with—(i)the tax year in which the business commences, for clause (a); or(ii)the tax year in which the extension of the undertaking is completed or the new unit commences production or operation, for clause (b).