题目

生态系统的结构包括 A．食物链和食物网            B．生态系统成分和营养结构 C．物质循环和能量流动        D．生产者、消费者、分解者和非生物物质能量 答案：BAs a mountain climber, Tom is used to risks. But he used to afraid of risks.A. take; be B. take; bein C. taking; be D. taking; being