Nanjing is the transportation hub in eastern China and the downstream Yangtze River area. Different means of transportation constitute a three-dimensional transport system that includes land, water and air. As in most other Chinese cities, public transportation is the dominant mode of travel of the majority of the citizens. As of October 2014, Nanjing had five bridges and two tunnels over the Yangtze River, which are tying districts north of the river with the city centre on the south bank.
How many tunnels does Nanjing have over the Yangtze River? (If the question is unanswerable, say "unanswerable")
two