James Cook, at the age of forty, was sent by England to explore the Pacific Ocean. On his third voyage, as captain in charge of two ships, the Discovery and the Revolution, he came upon a group of unknown islands which he named the Sandwich Islands after his friend, the Earl (伯爵) of Sandwich. Today the islands are known at the Hawaiian Islands.

    When Cook sailed into a protected bay (海湾) of one of the larger islands, the natives greeted him with curiosity and respect. Some historians said that the islanders welcomed him, believing he was the god, protector of peace and agriculture.

    The islanders were short, strong people with a very well-organized social system. The men fished and raised crops. The women prepared for the children and made clothing for the men and short skirts for the women.

已知数列{a_n}，如果$a_{1,}{a}_2-a_{1,}{a}_3-a_{2,}\cdots ,a_n-a_{n-1,\cdots }$是首项为1公比为2的等比数列，那么a_n=（   ）