光合作用实验中，若将叶的主脉切断，一切按规范操作，则发现叶片先端遇碘液未变蓝色。此实验说明（　　）

    One day Mrs. Wilson went shopping with Tracy and Ben. They went to the supermarket in the new shopping center.” Why do you buy things here?” Tracy wanted to know. “Because they are cheaper here than at the corner store near our home,” Mrs. Wilson said.” Help me check the prices, please.” The Wilsons were not rich and Mrs. Wilson was always careful with her money. She looked carefully after the prices of things. She bought lots of things in the supermarket. When they got home, the children said,“ We don't think you saved money by going to the supermarket.” “Of course I did,” Mrs. Wilson said. “ Everything was cheaper there.” “We know,” the children said,“ but we came home by taxi because we had too much to carry. The taxi fare was more than the money that you save! ” Mrs. Wilson added everything up. Her children were right.

堰塞湖是由于火山喷发、地震等原因引起山体岩石崩塌从而堵截山谷、河谷或河床后贮水而形成的湖泊，堵截物质称为堰塞体，往往不够稳定，易形成垮坝。读下图，回答：据图分析，导致图中堰塞湖形成的直接原因是(   )