Problem: Dog:

In 14th-century England, hound (from Old English: hund) was the general word for all domestic canines, and dog referred to a subtype of hound, a group including the mastiff. It is believed this "dog" type was so common, it eventually became the prototype of the category "hound". By the 16th century, dog had become the general word, and hound had begun to refer only to types used for hunting. The word "hound" is ultimately derived from the Proto-Indo-European word *kwon- "dog".

In what century did "hound" start to only apply to hunting dogs?
---
A: 16th

Problem: FA Cup:

The final has been played at the rebuilt Wembley Stadium since it opened, in 2007. The rebuilding process meant that between 2001 and 2006 they were hosted at the Millennium Stadium in Cardiff in Wales. Prior to rebuilding, the final was hosted by the original Wembley Stadium since it opened in 1923 (being originally named the Empire Stadium). One exception to this 78 year series of Empire Stadium finals (including five replays) was the 1970 replay between Leeds and Chelsea, held at Old Trafford in Manchester.

When did Old Trafford open?
---
A: unanswerable

Problem: Railway electrification system:

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.

In what decade did electrics use about 35% less fuel per ton-km.
---
A: unanswerable

Problem: Biodiversity:

Finally, an introduced species may unintentionally injure a species that depends on the species it replaces. In Belgium, Prunus spinosa from Eastern Europe leafs much sooner than its West European counterparts, disrupting the feeding habits of the Thecla betulae butterfly (which feeds on the leaves). Introducing new species often leaves endemic and other local species unable to compete with the exotic species and unable to survive. The exotic organisms may be predators, parasites, or may simply outcompete indigenous species for nutrients, water and light.

What animal can its feeding habits disturbed by the Prunus spinosa?
---
A:
leaves endemic and other local species unable to compete with the exotic species and unable to survive