生态系统中的物质流动是循环进行的，而能量流动是单向递减的。 [ ]
参考答案：对
I want to buy a comic book in that shop. But there is ______ left. [ ]
A. nothing
B. none
C. no one
D. nobody
A．mgtgθ
B．mgsinθ
C．μmg
D．μmg
mgtgθ
mgsinθ