编写VB程序,实现如下功能:在文本框Text1中输入一串字符,单击“统计”按钮Command1,统计每个英文字母(不区分大小写)出现的次数,找出出现次数最多的字母,在标签Label1中显示结果。运行效果如图所示。

实现上述功能的 VB 程序如下,请在划线处填入合适的代码。
Private Sub Command1_Click()
Dim i As Integer, n As Integer, max As Integer
Dim a(1 To 26) As Integer '数组a存放26个大小写字母出现的次数Dim st As String, ch As String
st = Text1.Text
For i = 1 To Len(st)
①_____
If ch >= "A" And ch <= "Z" Then n = Asc(ch) - Asc("A") + 1 a(n) = a(n) + 1
ElseIf ch >= "a" And ch <= "z" Then n =
②_____
a(n) = a(n) + 1
End If
Next i
max = a(1)
For i = 1 To 26
If
③_____Then max = a(i)
End If Next i st = ""
For i = 1 To 26
If max = a(i) Then
st = st + " " + Chr(Asc("A") + i - 1) End If
Next i
Label1.Caption = "出现次数最多的字母是:" & st & " " & "出现次数为:" & Str(max)
End Sub
(2)在文本框 Text1 中输入字符串“ABCEEEEffee333333”,运行程序时,标签 Label1 中输出结果为:出现次数最多的字母是:_____出现次数为:_____。