(b)such amount as is sufficient to reduce to nil the income (as computed before making the deduction under this section) of that previous year arising from the commercial exploitation [whether or not such commercial exploitation is as a result of the operations or development referred to in sub-section (2) of any mine or other natural deposit of the mineral or any one or more of the minerals in a group of associated minerals as aforesaid in respect of which the expenditure was incurred, whichever amount is less: