Real estate tycoon Donald Trump, with 24 percent, retired neurosurgeon Ben Carson, with 17 percent, and former Hewlett-Packard CEO Carly Fiorina, with 13 percent, take the top spots in Public Policy Polling's newest survey of GOP primary voters in the Hawkeye State.The rest of the candidates running for the Republican presidential nomination register in single digits, including Sens. Ted Cruz (Texas) and Marco Rubio (Fla.), at 8 percent each, and former Govs. Jeb Bush (Fla.) and Mike Huckabee (Ark.), at 6 percent apiece. Wisconsin Gov. Scott Walker, who dropped out of the GOP race Monday after falling dramatically in polls,...