This relay system allowed mail to criss-cross the f（） in record time.（－－the edge of land where people live and have built towns, beyond which the country is wild and unknown, especially in the western US in the 19th century）

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This relay system allowed mail to criss-cross the f（） in record time.（--the edge of land where people live and have built towns, beyond which the country is wild and unknown, especially in the western US in the 19th century）

正确答案：frontier

Tag：现代邮政英语时间：2022-01-30 21:00:45

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