Look, a group of students are playing the violin there, _______ their teacher as the conductor.

A．in front of them stand

B．in front of them stands

C．in front of whom standing

D．in front of whom stands

生态系统中营养物质再循环在人为状态下进行的途径是( )。

A．物质由动物排泄返回环境
B．人类利用化石燃料生产化肥，用海水制造淡水及对金属的利用
C．风化和侵蚀过程加上水循环携带营养元素进入生态系统
D．物质通过微生物分解有机物碎屑返回环境