单选题：Let's traverse a complete binary tree in level-order, and define

Let's traverse a complete binary tree in level-order, and define the level-order index of the root to be 1. Then for the $$i$$-th node visited, if its left child exists, then the index of this left child is___.
@[A](3)

A. $$2 i $$
B. $$i + 1$$
C. $$2 i - 2$$
D. $$2 ( i + 1 ) + 2$$



A.$$2 i $$
B.$$i + 1$$
C.$$2 i - 2$$
D.$$2 ( i + 1 ) + 2$$


答案：A