My elderly retired parents have been in USA on/off since Oct. 29th 2006 (Green Card issue date). Most of their trips outside the US are during the winter for less than 6 months, but once in 2010/11 they stayed out for nine months due to my grandmother’s illness. Now they are thinking to apply for Naturalization (US Citizenship). Will these 9 months absent from US prevent them to apply for citizenship? Please advice!