This task is about creating an unanswerable question based on a given passage. Construct a question that looks relevant to the given context but is unanswerable. Following are a few suggestions about how to create unanswerable questions:
(i) create questions which require satisfying a constraint that is not mentioned in the passage
(ii) create questions which require information beyond what is provided in the passage in order to answer
(iii) replace an existing entity, number, date mentioned in the passage with other entity, number, date and use it in the question
(iv) create a question which is answerable from the passage and then replace one or two words by their antonyms or insert/remove negation words to make it unanswerable.
Q: Passage: The bank is based in Frankfurt, the largest financial centre in the Eurozone. Its location in the city is fixed by the Amsterdam Treaty. The bank moved to new purpose-built headquarters in 2014 which were designed a Vienna-based architectural office named Coop Himmelbau. The building is approximately 180 metres (591 ft) tall and will be accompanied with other secondary buildings on a landscaped site on the site of the former wholesale market in the eastern part of Frankfurt am Main. The main construction began in October 2008, and it was expected that the building will become an architectural symbol for Europe. While it was designed to accommodate double the number of staff who operate in the former Eurotower, that building has been retained since the ECB took responsibility for banking supervision and more space was hence required.
A:
Where is the European Central bank unknown?