NY -- Standard&Poor's CreditWire 5/12/98 – Standard&Poor's today assigned its double-'A'-plus rating to Polk County, Iowa's G.O refunding bonds series 1998A due June 1, 1999-2011 and to the county's G.O refunding bonds series 1998B due June 1, 1999-2015, both dated May 1, 1998. Standard&Poor's also assigned its double-'A'-plus rating to the county's G.O. capital lien notes dated May 1, 1998 due June 1, 1999-2008. The bonds are scheduled to sell May 12, 1998. In addition, Standard&Poor's affirmed its double-'A'-plus rating on the county's G.O. bonds series 1989-1996 and taxable G.O. urban renew bonds series 1997. The outlook was revised to positive from stable. The rating reflects: -- A stable and diverse Des