Problems with generics

Today, I found a problem and I don't know what's going on. Please also ask Bo friends to answer.

Here's My Code:

 Package AAA;  Public class a{    }

 Package AAA;  Public class extends a{    }

 Package AAA;  Public class extends b{    }

 PackageAAA;Importjava.util.LinkedList;Importjava.util.List; Public classmain{ Public Static voidMain (string[] args[]) {List<?extendsB> eblist =NewLinkedlist<>(); List<?SuperB> sblist =NewLinkedlist<>(); A A=NewA (); b b=NewB (); C C=NewC ();        Eblist.add (a);    Sblist.add (c); }}

This eblist.add (a); Why did it get an error?

Problems with generics