有如下VB程序段：

Dims As String, c As String, st As String

Dim n As Integer, i As Integer

s ="20zj20sx7"：n=0：st=""

For i=1 To Len(s)

    c=Mid(s，i，1)

    If c>= "a" And c<= "z" Then

        st=Chr(Asc("a") *2+25-Asc(c))+st

    Else

        n=n*10+ Val(c)

    End If

Next i

st =st+str(n)

执行该程序段后，变量st的值是（    ）

A、chqa 20207

B、cgpz 47

C、zpgc 20207

D、zjsx 47