生产者、消费者、分解者和非生物成分这四个成分的结构，是生态系统的______结构，
参考答案：二级
With all the novels he was interested in , he left the library and went back home.
A．borrow
B．borrowing
C．to borrow
D．borrowed
2：1WBC和4：1WBC的含义？
