Suppose we have implemented the Stack ADT as a singly-linked-list with head and tail pointers and no sentinels. Which of the following best describe the running times for the functions push and pop, assuming there are O(n) items in the list, and that the bottom of the stack is at the head of the list (all pushing and popping occurs at the tail)? (a) O(1) for both functions. (b) O(n) for both functions. (c) O(1) for push and O(n) for pop. (d) O(n) for puh ad O(I) for pop. (e) None of these is the correct choice