((a)) ** General rule** In determining the amount of the deduction allowable to a lessee for exhaustion, wear and tear, obsolescence, or amortization in respect of any cost of acquiring the lease, the term of the lease shall be treated as including all renewal options (and any other period for which the parties reasonably expect the lease to be renewed) if less than 75 percent of such cost is attributable to the period of the term of the lease remaining on the date of its acquisition.

((b)) ** Certain periods excluded** For purposes of subsection (a), in determining the period of the term of the lease remaining on the date of acquisition, there shall not be taken into account any period for which the lease may subsequently be renewed, extended, or continued pursuant to an option exercisable by the lessee.