单选题：Which one of the following is **impossible** to be the insertion

Which one of the following is **impossible** to be the insertion sequence of the given binary search tree?



@[D](3)

A. 12, 18, 15, 8, 21
B. 12, 8, 18, 21, 15
C. 12, 18, 8, 21, 15
D. 12, 8, 15, 18, 21



A.12, 18, 15, 8, 21
B.12, 8, 18, 21, 15
C.12, 18, 8, 21, 15
D.12, 8, 15, 18, 21


答案：D