Read this and answer the question. If the question is unanswerable, say "unanswerable".

Boston's park system is well-reputed nationally. In its 2013 ParkScore ranking, The Trust for Public Land reported that Boston was tied with Sacramento and San Francisco for having the third-best park system among the 50 most populous US cities. ParkScore ranks city park systems by a formula that analyzes the city's median park size, park acres as percent of city area, the percent of residents within a half-mile of a park, spending of park services per resident, and the number of playgrounds per 10,000 residents.

Who ranks city park systems?
ParkScore