Why is doubly linked list not the answer to this question also why is circular linked list is the answet to it?

#1

The concatenation of two list can performed in O(1) time. Which of the following variation of linked list can be used?

Singly linked list

Doubly linked list

Circular doubly linked list

Array implementation of list

#2

Doubly linked list cannot be answer because we cannot find last element of a doubly linked list in O(1) time.
In a circular linked list, there is no last element. So two linked list can be merged in O(1) time

#3

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