如图是某地等高线地形图，AB段为河流，图中河流的大致流向为（   ）

D

    Our school is in Beijing. It isn't very big but it is very beautiful. There are a lot of trees in it. I'm in Class Two, Grade Seven. The walls of our classroom are white. There is a blackboard and some pictures on the wall. Our classroom is very clean （干净的）. There are 45 students in my class. Mr. White is our English teacher. He is from England. I like English very much. Mr. White is very friendly to us. He likes China and he is learning（学习）Chinese now. He can speak some Chinese. My father and mother are English teacher too. They often help me with my English. My classmates are very friendly. We are all friends. We love our school.

斜率为3，在y轴上的截距为4的直线方程是（　　）

如图所示的是人体结构层次流程图，请据图回答下列问题：