仔细阅读下列短文,并找出短文的主题词

仔细阅读下列短文,并找出短文的主题词
Visitors to the United States are often surprised to learn the distance from the east coast to the west coast.It is more than 3500 kilometres from the Atlantic Ocean on the east coast to the Pacific Ocean on the west.A jet plane crosses the continental United States in about five hours,but a traveler in an express train needs more than forty-eight hours to cross the country.