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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正确答案：frontier

Tag：时间：2023-10-07 15:10:34

上一篇：It was just a group of c（）riders who had proven their loyalty again and again.(–willing to work hard and give your time and energy to sth)
下一篇：They switched m（）every 10-12 miles.(–a horse that you ride on)