What is a dummy header in the context of linked list operations?

What is the worst-case time complexity of inserting 'n' elements into an initially empty linked list such that the linked list remains sorted?

What are the contents of an initially empty stack (from bottom to top) after performing the following operations? PUSH(A); PUSH(B); PUSH(C); POP; PUSH(D); POP; POP; PUSH(E); PUSH(F); POP;