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.
Example: Passage: In 1763, Spain traded Florida to the Kingdom of Great Britain for control of Havana, Cuba, which had been captured by the British during the Seven Years' War. It was part of a large expansion of British territory following the country's victory in the Seven Years' War. Almost the entire Spanish population left, taking along most of the remaining indigenous population to Cuba. The British soon constructed the King's Road connecting St. Augustine to Georgia. The road crossed the St. Johns River at a narrow point, which the Seminole called Wacca Pilatka and the British named "Cow Ford", both names ostensibly reflecting the fact that cattle were brought across the river there.
Example solution: Who owned Cuba after the Eight Years War?
Example explanation: This question appears to be relevant to the passage as both involves words such as 'Cuba' and 'War' which also exist in the passage. The passage mentions that "after the war, almost the entire Spanish population left, taking along most of the remaining indigenous population to Cuba". This information is not sufficient to conclude that which country owned cuba.

Problem: Passage: In the former Soviet Union, electric traction eventually became somewhat more energy-efficient than diesel. Partly due to inefficient generation of electricity in the USSR (only 20.8% thermal efficiency in 1950 vs. 36.2% in 1975), in 1950 diesel traction was about twice as energy efficient as electric traction (in terms of net tonne-km of freight per kg of fuel). But as efficiency of electricity generation (and thus of electric traction) improved, by about 1965 electric railways became more efficient than diesel. After the mid 1970s electrics used about 25% less fuel per ton-km. However diesels were mainly used on single track lines with a fair amount of traffic so that the lower fuel consumption of electrics may be in part due to better operating conditions on electrified lines (such as double tracking) rather than inherent energy efficiency. Nevertheless, the cost of diesel fuel was about 1.5 times more (per unit of heat energy content) than that of the fuel used in electric power plants (that generated electricity), thus making electric railways even more energy-cost effective.
Solution: In the US, electric traction became more energy-efficient than what?