What is the subject of the introduction?

Boston，is one of the largest and busiest cities in America. One of the reasons for its dynamism has been the success of its downtown area. This area incorporates a massive underground complex， linking the subway system with underground passages and shopping acacias， which in turn connect with a convention center and hotels. This urban concept is particularly appropriate because of the severe fluctuations in weather which Boston experiences. Annually，temperatures can vary by as much as 40 degrees. The underground zone ensures that the climate does not prevent people from gathering in the city center. Small and medium sized businesses have remained in the city rather than moving to the suburbs. The life has not drained out of the city，ensuring that Boston is a memorable place to visit.
Number 86.What is the subject of the introduction?
Number 87.Which of the following is NOT mentioned about the underground structure?
Number 88.What is the weather like in Boston?