地球资源指的是地球能提供给人类衣、食、住、行、医所需要的物质原料，也称为什么？
参考答案：自然资源
The old man, ____________ abroad for twenty years,is on the way back to his motherland[ ]
A.to work
B.working
C.to have worked
D.having worked
函数f（x）=lnx-x的单调减区间是______．
