【正确答案】 B

【答案解析】[听力原文]
The Universal Expo, also known as World's Fair, is one of the three largest events in the world, in terms of economic and cultural impact. However, compared with the other great events, the Modern Olympic Games, and the World Cup, the Expo has the longest history.
In 1851, the 1st Expo was held in London under the title "Great Exhibition of the Works of Industry of All Nations". At the Expo, participating countries will set up their national pavilions in distinctive architectural styles, with a view to attracting business people, and tourists all over the world. The cost for building such a pavilion could amount to millions of dollars. But it is often far outweighed by benefits and potential revenues thus generated.
Usually an Expo will last between three and six months. The character of World Exhibitions has developed through history. Three stages are identified, namely, the eras of industrialization, of cultural exchange, and of nation branding.
The 1st era covered roughly the period from 1800 to 1950. In these days, World Exhibitions were especially focused on trade and famous for the display of scientific and technological advancements.
The 2nd era stared with the 1939 New York World's Fair. From then on, World's Fairs became more strongly based on a specific theme of cultural significance. Technology and inventions remains important, but no longer as the principal subjects. The defining elements of the Expos were cross-cultural communications and exchanges.
Finally, in the early 1990s, countries started to use World Expo more widely and strongly as a platform to improve their national images.
A study shows that improving national image was the primary participation goal for 73% of the countries at Expo 2000. Today's World's Exhibitions embody elements of all three eras. They present new inventions, facilitate cultural exchange, and are used for city, region, and nation branding.

When was the 1st World Expo held?