Which factor accounts for this trend?

what factor? I can help you if you are clearer! :)

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Explanation:

(1) university of virginia: its a colonial heritage was founded in 1819 by declaration of independence author and former president thomas -charlottesville, virginia, united

(2) yellowstone national park is an american national park located in wyoming,montana and idaho and is a colonial heritage site.it was established by us congress and signed into law by president ulysses g grant on march 1,1872.yellowstone is usa's and world's first national park and a heritage site.the park is also known for its wildlife and its geothermal geysers.

(3) yosemite national park: : : colonial heritage site in north in western sierra nevada of central was designated a world heritage site in 1984..it is famous for its granite cliffs,waterfalls,clear streams,giant sequoia groves,lakes,mountains,glaciers and biological

(4) grand canyon national park: : colonial heritage site in north in north western arizona and is 15th site declared as national park..the park's central feature is the grand canyon, a gorge of the colorado river,which is also considered one of the wonders of the world.the park covers 1217262 acres.the grand canyon was designated a world heritage site in by unesco in 1979.

(5) taos pueblo

(6) redwood national park

(7) cahokia mounds

(8) wrangell-st. elias national park

(9) glacier bay national park

(10) kluane national park

(11) tatshenshini-alsek provincial park

(12) mammoth cave national park

(13) olympic national park

(14) great smoky mountains national park

(15) quebec city

(16) everglades national park

(17) lunenburg, nova scotia

(18) glacier national park

(19)chichen itza