国家植树节基金会向许多城市发起挑战，要求它们通过种植和保护更多的树木来赢得那面“美国树城”的旗帜。
The National Arbor Day Foundation has challenged many cities to earn its "Tree City USA" flag by planting and protecting more trees.
随着时间流逝，植树节已经成为美国的主要节日之一。
As years go by, Arbor Day has become one of the major holidays in America.
植树节起源于美国。
植树节根源于美国内布拉斯加州。
植树节根源于美国内布拉斯加州。
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