; (2) for valuation dates occurring during or after 2018, such mortality improvement rates shall not assume future mortality improvements at any age which are greater than .78 percent, and (3) plan sponsors shall be permitted to elect for the modifications under paragraphs (1) and (2) not to apply to a plan for valuation dates occurring during 2018. The Secretary shall by regulation modify the .78 percent figure in paragraph (2) as necessary to reflect material changes in the overall rate of improvement projected by the Social Security Administration. (b) Preservation of current law option Notwithstanding the modifications made under subsection (a), with respect to a plan for which substitute mortality tables are not used pursuant to Treas. Reg. section 1.430(h)(3)–2 for a plan year beginning during 2018, mortality tables determined in accordance with Treas. Reg. section 1.430(h)(3)–1 as in effect on December 31, 2017, may be used for purposes of applying the rules of section 430 of the Internal Revenue Code of 1986 for a valuation date occurring during 2018 if the plan sponsor— (1) concludes that the use of mortality tables determined in accordance with Treas. Reg. section 1.430(h)(3)–1 (without regard to any modification under this section) for the plan year would be administratively impracticable or would result in an adverse business impact that is greater than de minimis