The renewable fuel obligation deter- mined for a calendar year under clause (1) shall- (I) be applicable to refineries, blenders, and importers, as appropriate; (II) be expressed in terms of a volume percentage of transportation fuel sold or introduced into commerce in the United States; and (III) subject to subparagraph (C)(i), consist of a single applicable percentage that applies to all categories of persons specified in subclause (I). (C) Adjustments In determining the applicable percentage for a calendar year, the Administrator shall make adjustments- (i) to prevent the imposition of redun- dant obligations on any person specified in subparagraph (B)(ii)(I); and (ii) to account for the use of renewable fuel during the previous calendar year by small refineries that are exempt under paragraph (9). (4) Modification of greenhouse gas reduction percentages (A) In general The Administrator may, in the regulations under the last sentence of paragraph (2)(A)(i), adjust the 20 percent, 50 percent, and 60 percent reductions in lifecycle green- house gas emissions specified in paragraphs (2)(A)(i) (relating to renewable fuel), (1)D) (relating to biomass-based diesel), (1)(B)(i) (relating to advanced biofuel), and (1)(E) (re- lating to cellulosic biofuel) to a lower per- centage. For the 50 and 60 percent reduc- tions, the Administrator may make such an adjustment only if he determines that gener- ally such reduction is not commercially fea- sible for fuels made using a variety of feed- stocks, technologies, and processes to meet the applicable reduction.