(a) What values are returned during the following series of stack operations, if executed upon an initially empty stack? push(21), push(19), pop(), push(2), push(8), pop(), pop(), push(9), push(1), pop(), push(7), push(6), pop(), pop(), push(4), pop(), pop(). (Please provide the returned values in the format "value1, value2, value3...")

(b)What values are returned during the following series of queue operations, if executed upon an initially empty queue? enqueue(10), enqueue(3), dequeue(), enqueue(2), enqueue(15), dequeue(), dequeue(), enqueue(9), enqueue(1), dequeue(), enqueue(7), enqueue(6), dequeue(), dequeue(), enqueue(4), dequeue(), dequeue(). (Please provide the returned values in the format "value1, value2, value3...")