一个PCM系统中可用于作七号链路的时隙有（）个。
A.30
B.31
C.1
D.32
参考答案：B
如果绿色植物被大量砍伐，大气中的二氧化碳就会急剧增加，并能引起温室效应，使地球环境不断恶化。这说明绿色植物能( )
A：调节碳氧平衡 B．过滤空气尘埃
C．降低环境噪声 D．涵养土壤水源
All the students went to Lushan during the summer holidays except ones who had already been there.
