Heavy snow and low temperatures caused further damage across northern Europe on Monday--stranding  travelers,snarling traffic and shutting schools. More than 1,000 flights at Germany's main airports in Frankfurt,Munich and Berlin were _ and many more delayed after up to 40 cm of fresh snow covering the country.Some 500 passengers slept at Frankfurt airport.Airlines advised passengers to take trains if possible. In the United Kingdom,British Airways said the severe weather continued to cause great damage to operations and further travel.Only one of two runways at London Heathrow,the world's busiest international airport,was operating after the snowstorm.Other UK airports were open,but many flights were canceled,and many passengers spent a second night at an airport terminal . Northern France was also covered by heavy snow.Air travel was reduced at Paris' two main airports,with Orly airport shutting down. In Poland,hard hit by the cold weather,six people froze to death on Sunday night,raising the death toll to 114 in the last month.Heavy snow stopped Warsaw traffic again on Monday.Warsaw airport was open but was receiving far fewer passengers than usual because of flight cancellations in western Europe. The world's busiest international airport is  _  .
Berlin airport Paris airport Warsaw airport London Heathrow airport