泛型的问题

package aaa;
public class A
{
    
}

package aaa;
public class B extends A
{
    
}

package aaa;
public class C extends B
{
    
}

package aaa;

import java.util.LinkedList;
import java.util.List;

public class Main
{
    public static void main(String[] args[]){
        List<? extends B> eBList = new LinkedList<>();
        List<? super B> sBList = new LinkedList<>();
        A a = new A();
        B b= new B();
        C c = new C();
        eBList.add(a);
        sBList.add(c);
    }
}