Input: Article: The London Underground in England is one of the few networks that uses a four-rail system. The additional rail carries the electrical return that, on third rail and overhead networks, is provided by the running rails. On the London Underground, a top-contact third rail is beside the track, energized at +420v DC, and a top-contact fourth rail is located centrally between the running rails at −210v DC, which combine to provide a traction voltage of 630v DC. London Underground is now upgrading its fourth rail system to 750v DC with a positive conductor rail energised to +500v DC and a negative conductor rail energised to -250v DC. However, many older sections in tunnels are still energised to 630v DC. The same system was used for Milan's earliest underground line, Milan Metro's line 1, whose more recent lines use an overhead catenary or a third rail.

Now answer this question: What type of system does London Underground use?

Output: four-rail system

Input: Article: The Soviet Union created the Eastern Bloc of countries that it occupied, annexing some as Soviet Socialist Republics and maintaining others as satellite states that would later form the Warsaw Pact. The United States and various western European countries began a policy of "containment" of communism and forged myriad alliances to this end, including NATO. Several of these western countries also coordinated efforts regarding the rebuilding of western Europe, including western Germany, which the Soviets opposed. In other regions of the world, such as Latin America and Southeast Asia, the Soviet Union fostered communist revolutionary movements, which the United States and many of its allies opposed and, in some cases, attempted to "roll back". Many countries were prompted to align themselves with the nations that would later form either NATO or the Warsaw Pact, though other movements would also emerge.

Now answer this question: What actions of the Soviet Union did The United States oppose?

Output: the Soviet Union fostered communist revolutionary movements,

Input: Article: It was principally the widespread introduction of a single feature, the pointed arch, which was to bring about the change that separates Gothic from Romanesque. The technological change permitted a stylistic change which broke the tradition of massive masonry and solid walls penetrated by small openings, replacing it with a style where light appears to triumph over substance. With its use came the development of many other architectural devices, previously put to the test in scattered buildings and then called into service to meet the structural, aesthetic and ideological needs of the new style. These include the flying buttresses, pinnacles and traceried windows which typify Gothic ecclesiastical architecture. But while pointed arch is so strongly associated with the Gothic style, it was first used in Western architecture in buildings that were in other ways clearly Romanesque, notably Durham Cathedral in the north of England, Monreale Cathedral and Cathedral of Cefalù in Sicily, Autun Cathedral in France.

Now answer this question: What is another example of a Gothic style element that was able to be incorporated because of technological innovation?

Output:
traceried windows