This question is about the deque ADT that is described in the textbook. Suppose you perform the following sequence of operations on an empty deque, which stores integers. What is the output issued by the last operation? Choose one. addFirst(10), addFirst(20), addLast(30), last( ), removeLast( ), first( ), isEmpty( ), removeLast( ), removeFirst( ), addLast(40), addLast(50), first( )
a. 20
b. 50
c. 30
d. 40