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The  Kolkata Metro  or Calcutta Metro ( Bengali : ) is a mass rapid transit sy  stem ser  ving the city of Kolkata and the districts of South 24 Parganas and North 24 Parganas in Indian state of West Bengal .
 city of headquarters: Kolkata Metro, stem ser
Given a phrase that describes the relationship between two words, extract the words and the lexical relationship between them. The output format should be "relation1: word1, word2; relation2: word3, word4". 
Option: cities of residence, top members employees, members, alternate names, origin, state or provinces of residence, title of person, spouse, city of headquarters, NA, founded by, state or province of headquarters, employee of, countries of residence, country of birth, founded, subsidiaries, country of headquarters 
Text: {0} 
Answer:
 city of headquarters: Kolkata Metro, stem ser