The　most　important　use　of　drifting　(漂流)　bottles　is　to　find　ocean　currents.　When　the　position　and　direction　of　currents　are　known,　ships　can　use　the　forward　movement　of　a　current　or　stay　away　from　currents　that　would　carry　them　off　their　course.　Benjamin　Franklin　was　one　of　the　first　to　use　bottles　in　the　study　of　currents.　He　wondered　why　British　mall　ships　needed　a　week　or　two　longer　than　U.S.　ships　needed　in　order　to　cross　the　Atlantic　Ocean.　Franklin　thought　the　Gulf　Stream　(墨西哥湾流)　might　explain　this　difference.Franklin　talked　with　captains　of　U.S.　ships.　He　found　that　they　knew　each　turn　of　the　Gulf　Stream.　They　used　the　current　in　every　possible　way.　From　his　talks　with　the　captains.　Franklin　made　his　first　map　of　the　Gulf　Stream.　Then　he　checked　his　map　by　using　sealed　(密封的)　bottles.　The　map　that　he　finally　made　is　still　used,　with　only　a　few　changes,　today.Why　are　drifting　bottles　used?

A.
To determine the position of a ship.

B.
To find the direction of a current.


C.
To predict the direction of a ship.

D.
To carry message across the ocean.

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