已知字符“a ”的 ASCII 码值为 97，有如下 Python  程序段：

que=[""]*20

head,tail= 0,0

for i in range(3):

    que[tail]=chr(97+i)

    tail+= 1

st=["b","c","d","a"]

top=3

while head < tail and top > - 1:

    if st[top]==que[head]:

        head+= 1

    else:

        que[tail] = st[top]

        tail+= 1

    top-= 1

print(que[head:tail])

执行该程序段，则输出的结果是(  )

A、['c', 'd', 'c']

B、['c', 'c', 'd']

C、['c', '', 'd']

D、['c', 'd']