A few months ago, millions of people in London heard alarms all over the town. The Emergency Services, the Fire Departments, the Police, hospitals, and ambulances stood by, ready to go into action. In railway underground stations, people read notices and maps that told them where to go and what to do in the emergency. This was Exercise Flood Call, to prepare people for a flood emergency. London wasn't flooded. In 1928, people living in Westminster, the heart of London, drowned in floods. And in 1953, one hundred people, living on the eastern edge of the London suburbs were killed, again, in the floods. At last, Greater London Council took actions to prevent this disaster from happening again.  Though a flood wall was built in the 1960s, Londoners still must be prepared for the possible disaster. If it happens, 50 underground stations will be under water. Electricity, gas and phone services will be out of action. Roads will be drowned. It will be impossible to cross any of the bridges between north and south London. Imagine: London will look like the famous Italian city, Venice. But this Exercise Flood Call didn't cause panic among Londoners. Most people knew it was just a warning. One lady said, "It's a flood warning, isn't it. ? The water doesn't look high to me." The purpose of the Exercise Flood Call is to   _  .
train the civil service for the flood prevent the flood from happening fill people in London with alarm prepare people against floods