One kind of vacation that many Americans enjoy is camping. Each summer 46._____ (million) of Americans drive to the countryside where they find places 47._____ (camp). The 48.______ (nation) parks, many of which are in the mountains, are favorite camping places. Campers enjoy the 49. _______( 新鲜的 ) air, the lakes and the forests which they find in these parks. Campers hike, swim and fish. They can also find many 50. k______ of animals and plants in the parks. Most campers have trailers( 拖车 ) which they drive or pull behind 51._______ (they) cars to their campsites. Trailers are like houses on wheels. They have many conveniences which people have in their homes, such 52._______ electricity and hot water. But most 53. _______ (camp) don’t have trailers. They camp in tents which they set 54.________ in their campsites. Campers in tents don’t have the conveniences that campers in trailers have. Tent campers enjoy a 55. _____ (simply) life.