48. Consider the implementation of the List class in your textbook. The insert function is shown in the code snippet below. What should be the code for the missing operation? void List: :insert (Iterator iter, string s) if (iter.position NULL) push_back (s); return; Node * after = ?ter.posit? n ; Node * bef re = after->previ us ; N de * new n de new N de (s) ; new node-previous before; new node->next = after; // missing operation if (beforeNULL) // Insert at beginning first = new n de ; else before->next new node a) b) c) d) after->previous= new-node ; after->next = new n de ; new-node = after->previous ; new node = after->next