有如下VB程序：

n = 4

For i = 1 To 25

    j = 1: a(j) = a(j) + 1

    Do While a(j) = 4

        a(j) = 0

        j = j + 1

        a(j) = a(j) + 1

    Loop

Next i

For i = 1 To n

    s = Str(a((i + 1) Mod 4 + 1)) + s

Next i

数组a(1)至a(4)初值均是0，执行该程序后，s的值是（     ）

A、"1012"

B、"2101"

C、"0121"

D、"1210"