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: Nintendo of America took the same stance against the distribution of SNES ROM image files and the use of emulators as it did with the NES, insisting that they represented flagrant software piracy. Proponents of SNES emulation cite discontinued production of the SNES constituting abandonware status, the right of the owner of the respective game to make a personal backup via devices such as the Retrode, space shifting for private use, the desire to develop homebrew games for the system, the frailty of SNES ROM cartridges and consoles, and the lack of certain foreign imports.
What did owners of the respective game consider emulators?

Passage: Much of YouTube's revenue goes to the copyright holders of the videos. In 2010 it was reported that nearly a third of the videos with advertisements were uploaded without permission of the copyright holders. YouTube gives an option for copyright holders to locate and remove their videos or to have them continue running for revenue. In May 2013, Nintendo began enforcing its copyright ownership and claiming the advertising revenue from video creators who posted screenshots of its games. In February 2015, Nintendo agreed to share the revenue with the video creators.

What did Nintendo start enforcing in 2015?