The students were having their chemistry class. Miss Li was telling the children what water was like. After that, she asked her students, “What's water?” No one spoke for a few minutes. Miss Li asked again, “Why don't you answer my question? Didn't I tell you what water is like?”
   Just then a boy put up his hand and said, “Miss Li, you told us that water has no color and no smell. But where can we find such kind of water？The water in the river behind my house is always black and it has a bad smell.” Most of the children agreed with him.
   “I'm sorry, children,” said the teacher. “Our water is getting dirtier and dirtier. That's a problem.”

在抛物线$y^2=2px\left(p>0\right)$上，横坐标为4的点到焦点的距离为5，则该抛物线的准线方程为(     )