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：_____。
