21.细节理解题。根据Yellowstone National Park部分“The almost3,500-square-mile park includes parts of thestates of Wyoming,.Idaho,and Montana..”可知,黄石公园横跨三个州。故选A项。22.细节理解题。根据Crater of Diamonds State Park部分“Visitors can search the37.5-acre site for diamonds and ifyou find it you keep it.”可知,游客在这里可以寻找钻石。故选B项。23.细节理解题。根据Hawaii Volcanoes National Park部分“You can see how the Hawaiian Islands were born andwatch the Big Island get bigger.”可知,在Hawai'i Volcanoes National Park,游客可以了解岛屿是如何形成的,并看到它变得越来越大的过程。故选D项。