NEW YORK -- Visitors to the museum at the national 9/11 memorial could be required to pay an entrance fee of about $20, though victims' relatives will always be able to enter for free, the president and CEO of the memorial foundation said Thursday.Joe Daniels, the president and CEO of the National September 11 Memorial & Museum, told City Council members at a hearing that once the museum opens in September 2012, it must generate enough income to maintain itself and the memorial.