The focus of today's class is on an important element in the development of United States cities, something the modem cities need to survive: efficient transportation. The rapid growth of cities in 19th century created a serious problem. Growth obviously meant greater distances between people's home and their work places. People could no longer rely on walking. By the mid 19th century, some United States cities had used railways drawn by horses, but they were very slow. City planners understood that without improved transportation cities could not grow any larger and they welcome the invention of system of trolley cars on cables, powered by steam engines. These cables were able to pull cars up to the steep hills of San Francisco and so other large cities installed similar systems in 1880s. But although faster than horses, these cable cars still weren't fast enough to solve the transportation problems of United States cities. Our real breakthrough came thanks to electricity. Electric trolley move twice as fast as the old horse drawn railways and they were clean. They designers of electric trolley came up with the new control system that allowed each part to be powered independently and have its own breaks, what's
safer and more efficient than cable cars. Electricity also helped the next great advance in urban transportation, the subway. It solved one problem that the early English subways suffered from, unbearable smoke of steam engines that made breathing in the subway tunnels almost impossible.
解析
What is the talk mainly about?
主旨题。根据The focus of today’s class is on an important element in the development of United States
cities．something the modern cities need to survive：efficient transportations可知本文谈论的是城市交通，结合全文。应该是城市交通的发展对城市发展的促进作用。因此，正确答案是B。