单项选择题
Set the initial state of stack S and queue Q to be empty. Elements e1, e2, e3, e4, e5, and e6 push into the stack S in sequence. When an element is popped, it push into the queue Q immediately,if the pop sequence of 6 elements is e2, e4, e3, e6, e5, e1, then the capacity of stack S should be at least( ).
A.A 3
B.B 6
C.C 4
D.D 2