下列关于能量和物质循环正确的说法是( )。

A．能量不断地沿着生产者、消费者、分解者及环境方向逐级流动
B．生态系统的能量在各营养级间往返流动
C．物质循环是指各种化学物质在生物和非生物之间的循环运转
D．循环”一词意味着这些化学物质可以被永远重复利用

If you must jump from a ship，your posture should include（）.

A.holding down the life preserver against the chest with one arm crossing the other，covering the mouth and nose with a hand，and feet together

B.knees bent and held close to the body with both arms around legs

C.body straight and arms held tightly at the sides for feet first entry into the water

D.both hands holding the life preserver below the chin，with knees bent，and legs crossed