阅读下面短文，在空白处填入1个适当的单词或括号内单词的正确形式。

Although parks of all sizes and types exist at any level, the national parks, in particular, tend （1） （catch） our attention because of their large size and variety. They are （2） （treasure）of American heritage（遗产）. How did the national park system come about? On a cool, starry night in mid-September 1870, four men relaxed before a campfire along the Firehole River in （3） is now northwestern Wyoming. They （4） （be） part of a 15-member exploring party that had spent almost five awesome weeks in witness of the natural beauties there.

What should （5） （do）with such a beautiful place? They wondered out loud. This area, with （6） （it）unique and breathtaking natural beauty, must be well preserved （7） all people of the nation to enjoy — as a national park. They all agreed and vowed（承诺）to promote the idea at the （8） （complete）of their journey. Their promotional work paid off. Two years later, 18 years before Wyoming became a state, Yellowstone became the first national park in the United States and the world. Yellowstone was the （9） （large）United States national park — 2.2 million acres — until Wrangell-Saint Elias in southern Alaska, （10） became a national monument in 1978, took the honors as a national park in 1980 with 12.3 million acres.