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?



@[B](3)

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



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


答案：B