This task is about reading the given passage and construct a question about the information present in the passage. Construct a question in such a way that (i) it is unambiguous, (ii) it is answerable from the passage, (iii) its answer is unique (iv) its answer is a continuous text span from the paragraph. Avoid creating questions that (i) can be answered correctly without actually understanding the paragraph and (ii) uses same words or phrases given in the passage.

In 121 BC, Han forces expelled the Xiongnu from a vast territory spanning the Hexi Corridor to Lop Nur. They repelled a joint Xiongnu-Qiang invasion of this northwestern territory in 111 BC. In that year, the Han court established four new frontier commanderies in this region: Jiuquan, Zhangyi, Dunhuang, and Wuwei. The majority of people on the frontier were soldiers. On occasion, the court forcibly moved peasant farmers to new frontier settlements, along with government-owned slaves and convicts who performed hard labor. The court also encouraged commoners, such as farmers, merchants, landowners, and hired laborers, to voluntarily migrate to the frontier.
Who made up a majority of the people on the frontier?

A brief shoot at London's City Hall was filmed on 18 April 2015, while Mendes was on location. On 17 May 2015 filming took place on the Thames in London. Stunt scenes involving Craig and Seydoux on a speedboat as well as a low flying helicopter near Westminster Bridge were shot at night, with filming temporarily closing both Westminster and Lambeth Bridges. Scenes were also shot on the river near MI6's headquarters at Vauxhall Cross. The crew returned to the river less than a week later to film scenes solely set on Westminster Bridge. The London Fire Brigade was on set to simulate rain as well as monitor smoke used for filming. Craig, Seydoux, and Waltz, as well as Harris and Fiennes, were seen being filmed. Prior to this, scenes involving Fiennes were shot at a restaurant in Covent Garden. Filming then took place in Trafalgar Square. In early June, the crew, as well as Craig, Seydoux, and Waltz, returned to the Thames for a final time to continue filming scenes previously shot on the river.
Which bridges were shut down because of filming?

In general, German bombers were likely to get through to their targets without too much difficulty. It was to be some months before an effective night fighter force would be ready, and anti-aircraft defences only became adequate after the Blitz was over, so ruses were created to lure German bombers away from their targets. Throughout 1940, dummy airfields were prepared, good enough to stand up to skilled observation. A number[clarification needed] of bombs fell on these diversionary ("Starfish") targets.

Anti-aircraft defenses became better after what?