what are the invariant of the stack class (array version) a. the number of items in the stack is stored in the member variable used. b. the items in the stack are stored in a partially filled array called data, with the bottom of the stack at data[0], the next entry at data[1], and so on to the top of the stack at data[used-1]. c. the items in the stack are stored in a linked list, with the top of the stack stored at the head node, down to the bottom of the stack at the tail node. d. the member variable top ptr is the head pointer of the linked list of items.