—Where is Zhang Ming?

—Oh,he ______to Beijing to see his parents and he'll be back tomorrow.[ ]

A. has gone              

B. has been

C. had gone              

D. had been

下图是生态系统中碳、硫等物质循环的简图，请分析回答下列问题：

如果该生态系统中次级消费者增重2kg，至少需要生产者提供（）kg。