I__________sooner but I didn't know that they were waiting for me.   [ ]

A. had come                        

B. was coming  

C. would come                      

D. would have come

读下方黄河水系图，回答下列问题：

（1）图中①是______河，②是______河；它们都位于黄河的_______游。

（2）从干湿地区看，黄河下游流经________地区。

（3）黄河下游最大的水利枢纽是__________，其主要作用是__________。

（4）黄河下游除了“地上河”的忧患之外，近些年尤为突出的问题是_____________________________。

     应立即采取的措施是_____________________________________________。