生态系统的物质循环发生在（）
A.生物群落中
B.生物群落与无机环境间
C.生产者与消费者间
D.生产者与分解者间
参考答案：B
Had your father seen the doctor earlier, he ____.
A．would have died
B．would not have died
C．would not die
D．has not died
因为年龄相近、支持某个球队、同一所大学毕业、政治观点相同等原因形成的群体是（）群体。
A.工作型
B.友谊型
C.学习型
D.利益型
