单选题 Formulated in 1823, the Monroe Doctrine asserted that the Americas were no longer open to European colonization.

【正确答案】 A

【答案解析】assert：宣称；断言。与state firmly(坚定地说)意思相近。argue light-mindedly：轻率地争辩。think seriously：严肃地认为。announce regrettably：令人遗憾地宣布。