题目

阅读单选 Many cultures have stories of talking trees that give advice as well as warnings to people. Alexander the Great and Marco Polo were said to have visited such a tree in India. And in some modern stories, such as the film Avatar, trees can communicate with animals and people.New research has found something amazing: it seems that plants can communicate after all.It has been known for some time that plants use chemicals to communicate with each other. This happens when a plant gets attacked by insects. The plant gives out chemicals from the leaves that are being eaten. This is like a如图，一重4牛的木箱静止在斜面上，请用给定标度作出其所受重力的图示．