执行下面程序,单击按钮CmdRun,窗体上显示的第一行结果是（）,UBound(a)的值为（）,其中a(1)的值为（）.

Option Explicit

Private Sub CmdRun_Click()

Dim St As String,i As Integer

Dim a() As String,j As Integer,k As Integer

St="abcd"

Call Sub1(St)

Print St

For i=1 To Len(St)

For j=i+1 To Len(St)

If Mid(St,i,1)=Mid(St,j,1) Then Exit For

Next j

If j>Len(St) Then

k=k+1

ReDim Preserve a(k)

a(k)=Mid(St,i,1)

Print "a(";k;")=";a(k)

End If

Next i

End Sub

Private Sub Sub1(S As String)

Dim i As Integer

For i=1 To Len(S)\2

Mid(S,i,1)=Mid(S,Len(S)-i+1,1)

Next i

End Sub