69001
Some systems cover entire cities such as Stockholm or Flensburg , using a network of large 1000 " mm diameter primary pipes linked to secondary pipes – 200 " mm diameter perhaps , which in turn link to tertiary pipes of perhaps 25 " mm diameter which might connect to 10 to 50 houses .
 Stockholm, Flensburg
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: media, plant, vehicle, astronomical object, time, disease, animal, location, person, organization, mythical figure, food, biology, geographical phenomenon, instrument, event 
Text: {0} 
Answer:
 Stockholm, Flensburg