1. (2021高二上·瑞安月考) 某Python程序代码如下：

s='t1Hr2a3'

s1='' ; s2=''; i = 0

while i < len(s):

    if '0'<=s[i]<='9': 

        s1 = s1 + s[i] 

    elif 'a'<=s[i]<='z':

        s2 = s[i] + s2 

        i = i + 1

        ans = s2 + s1

print(ans)

程序执行后，输出的结果是(      )