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.

Passage: There has, however, been interest among railroad operators in returning to DC use at higher voltages than previously used. At the same voltage, DC often has less loss than AC, and for this reason high-voltage direct current is already used on some bulk power transmission lines. DC avoids the electromagnetic radiation inherent with AC, and on a railway this also reduces interference with signalling and communications and mitigates hypothetical EMF risks. DC also avoids the power factor problems of AC. Of particular interest to railroading is that DC can supply constant power with a single ungrounded wire. Constant power with AC requires three-phase transmission with at least two ungrounded wires. Another important consideration is that mains-frequency 3-phase AC must be carefully planned to avoid unbalanced phase loads. Parts of the system are supplied from different phases on the assumption that the total loads of the 3 phases will even out. At the phase break points between regions supplied from different phases, long insulated supply breaks are required to avoid them being shorted by rolling stock using more than one pantograph at a time. A few railroads have tried 3-phase but its substantial complexity has made single-phase standard practice despite the interruption in power flow that occurs twice every cycle. An experimental 6 kV DC railway was built in the Soviet Union.
Railroad operators have shown little interest in returning to what?

Passage: Bermuda's pink sand beaches and clear, cerulean blue ocean waters are popular with tourists. Many of Bermuda's hotels are located along the south shore of the island. In addition to its beaches, there are a number of sightseeing attractions. Historic St George's is a designated World Heritage Site. Scuba divers can explore numerous wrecks and coral reefs in relatively shallow water (typically 30–40 ft or 9–12 m in depth), with virtually unlimited visibility. Many nearby reefs are readily accessible from shore by snorkellers, especially at Church Bay.
What are located along the shore of the coral reefs?

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.

Where is the European Central bank unknown?