How to call protected function?

How to call function lmn() without touching class B from class A

 class A extends B{
     public function abc(){
         return "abc";
     }
     ...
 }

 class B{
     public function xyz(){
         return "xyz";
     }
     ...
 }

 class C{
     protected function lmn(){
         return "lmn";
     }
     ...
 }

please guide me for this