听力原文： Police in London have warned people to remain cautious following the bomb attacks during Thursday morning's rush hour on the city's transport system. The attacks are believed to have killed at least fifty people and injured around seven hundred more. The first attack came just before nine in the morning on a train close to the main station in the city's financial district minutes later the worst incident occurred: a bomb exploded in a deep underground line, killing more than twenty people. A third bomb on another underground train tore a hole through a tunnel wall, throwing broken pieces onto a nearby track and involving a further two trains. The fourth explosion raised the roof off a bus. The attacks were timed to cause maximum disruption during the morning rush hour, and with the underground network still shut down the city's workers are faced with a long walk home. The bomb attacks happened in London with a casualty of