单选题：If keys are pushed onto a stack in the order **abcdef**, then it

If keys are pushed onto a stack in the order **abcdef**, then it's impossible to obtain the output sequence ( ). @[D](3)

A. fedcba
B. bcafed 
C. dcefba
D. cabdef 




A.fedcba
B.bcafed
C.dcefba
D.cabdef


答案：D