【单选题】

There are warm tropical regions all over the globe, but only the Indians of the SouthAmerican rain forests have formed the habit of sleeping in the open air. Long before they made painful acquaintance (相识) withEuropeans, they had invented something that was unique on earth: the hammock.

Nobody really knows who first had the bright idea of making sleeping in the air the symbol of untroubled rest. The Indians see the hammock as a "gift of heaven (上天)", something given to them a very long time ago.
In it the Indians pass away hot noon hours, napping or chatting. Swinging it to and fro (来回摆动) creates a cooling breath of air and keeps away insects. They work and play in hammocks, are born and die there.
Hung like a suspension bridge between heaven and earth, a hammock is dry while the soil is damp and is safe from most wild animals.
Hammocks have the advantage over beds in that they are easy to transport and take up very little space when they have been rolled up (卷起). Indians never go on a journey without their hammocks, not even to their plantations.
The word hammock in this passage means ______.
A、a suspension cage which can be swung to and fro in the air
B．a seat hanging by two ropes from the branch of a tree
C．a suspension bridge in the SouthAmerican rain forests
D．a net hung between two supports and used as a bed