instruction:
This task is about writing a correct answer for the reading comprehension task. Based on the information provided in a given passage, you should identify the shortest continuous text span from the passage that serves as an answer to the given question. Avoid answers that are incorrect or provides incomplete justification for the question.
question:
Passage: Gibraltar opened its first comprehensive school in 1972. Between the ages of 12 and 16 two comprehensive schools cater for girls and boys separately. Students may also continue into the sixth form to complete their A-levels. Question: When was the first comprehensive school opened in Gibraltar?
answer:
1972


question:
Passage: AA gunnery was a difficult business. The problem was of successfully aiming a shell to burst close to its target's future position, with various factors affecting the shells' predicted trajectory. This was called deflection gun-laying, 'off-set' angles for range and elevation were set on the gunsight and updated as their target moved. In this method when the sights were on the target, the barrel was pointed at the target's future position. Range and height of the target determined fuse length. The difficulties increased as aircraft performance improved. Question: What was the problem with AA gunnery?
answer:
successfully aiming a shell to burst close to its target's future position


question:
Passage: The railroads opened up the Great Plains for settlement, for now it was possible to ship wheat and other crops at low cost to the urban markets in the East, and Europe. Homestead land was free for American settlers. Railroads sold their land at cheap rates to immigrants in expectation they would generate traffic as soon as farms were established. Immigrants poured in, especially from Germany and Scandinavia. On the plains, very few single men attempted to operate a farm or ranch by themselves; they clearly understood the need for a hard-working wife, and numerous children, to handle the many chores, including child-rearing, feeding and clothing the family, managing the housework, feeding the hired hands, and, especially after the 1930s, handling paperwork and financial details. During the early years of settlement, farm women played an integral role in assuring family survival by working outdoors. After approximately one generation, women increasingly left the fields, thus redefining their roles within the family. New technology including sewing and washing machines encouraged women to turn to domestic roles. The scientific housekeeping movement, promoted across the land by the media and government extension agents, as well as county fairs which featured achievements in home cookery and canning, advice columns for women regarding farm bookkeeping, and home economics courses in the schools. Question: what mainly opened the great plains for settlement?
answer:

railroads