单选题：It remains a puzzle about why our outermost planet exists.

题目内容：

It remains a puzzle about why our outermost planet exists. A．problem
B．mystery
C．question
D．point

参考答案：

答案解析：

Formulated in 1823, the Monroe Doctrine asserted that the A

Formulated in 1823, the Monroe Doctrine asserted that the Americas were no long

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