NEW YORK, NY - DECEMBER 09:  Prince William, Duke of Cambridge and Catherine, Duchess of Cambridge look at a portion of the North Tower antenna during a visit the National September 11 Memorial & Museum on December 09, 2014 in New York City.  The couple, who are traveling without their son Prince George, are on a three-day US east coast visit. This is the Duke and Duchess' first official visit to New York City. (Photo by Doug Mills - Pool/Getty Images)