有如下 Python 程序段：

s = input('请输入一串小写字母') head =  0; tail = 0; top = -1  s1 = [""]*((len(s)+1)//2)

s2 = [""]*(len(s)//2)

for i in range(len(s)):

    if i % 2 == 0:

        s1[tail] = s[i]

        tail += 1

    else:

        top += 1

        s2[top] = s[i] x = ""

while head < tail and top > -1:

    x = s1[head] + x

    head += 1

    x = x + s2[top]

    top -= 1 print(x)

执行该程序段，输入字符串“abcdefg”，则输出的结果是（    ） 

A、acegbdf

B、acegfdb

C、gecafdb

D、ecafdb