Read this: Many historical sites exist throughout the city, including 59 properties listed on the National Register of Historic Places. Of these, nine are among the 60 U.S. National Historic Landmarks in Connecticut. The New Haven Green, one of the National Historic Landmarks, was formed in 1638, and is home to three 19th-century churches. Below one of the churches (referred to as the Center Church on-the-Green) lies a 17th-century crypt, which is open to visitors. Some of the more famous burials include the first wife of Benedict Arnold and the aunt and grandmother of President Rutherford B. Hayes; Hayes visited the crypt while President in 1880. The Old Campus of Yale University is located next to the Green, and includes Connecticut Hall, Yale's oldest building and a National Historic Landmark. The Hillhouse Avenue area, which is listed on the National Register of Historic Places and is also a part of Yale's campus, has been called a walkable museum, due to its 19th-century mansions and street scape; Charles Dickens is said to have called Hillhouse Avenue "the most beautiful street in America" when visiting the city in 1868.

According to the National Register of Historic Places how many historical sites are there in New Haven? 
What is the answer? (If it cannot be answered, return "unanswerable")
59