The Thames was vital to the defense of Great Britain during World War II. In 1940, the German Air Force started bombing London. German pilots followed the river up into the center of the city, where they dropped their bombs. The first attack destroyed almost all of the docks in London’s port and killed hundreds The British also used the Thames to help defend their city. Water was pumped out of the Thames to put out fires started by the bombs and volunteers patrolled the river to ensure that the German ships didn’t enter the Port of London. Supplies and troops often traveled up and down the Thames when bombings obstructed the roads.
In paragraph six, the term "vital" means
