In a proceeding under subsection (a) of section 16-19, the Public Utilities Regulatory Authority may approve a modification of the existing business to residential pricing ratio for basic exchange service of a telephone company toward the then-current national average business to residential pricing ratio, as determined by the authority, but shall not approve a modification which brings such ratio below two and seven-tenths to one prior to June 30, 1994, and shall not approve a modification which brings such ratio below two and five-tenths to one or the then-current national average business to residential pricing ratio, as determined by the authority, whichever is higher, prior to June 30, 1995.