编写VB程序,实现如下功能:单击“启动”按钮Command1,在列表框List1内每隔2秒输出一个由30个大、小写字母组合成的字符串,运行界面如图所示,程序代码如下。
(字母A和字母a的ASCII码值分别为65、97)

(1)该程序除窗体外使用了________(填数字)个对象。
(2)为实现上述功能,请在划线处填入合适的代码。
Private Sub Command1_Click()
Timer1.Enabled = _________________________'①
End Sub
Private Sub Timer1_Timer()
Dim s As String, c As String
Dim a As Integer, e As Integer, h As Integer
Randomize
s = ""
For a = 1 To 30
e = Int(Rnd() * 52 + 1)
If e <= 26 Then
h = e + 64
Else
h = _________________________'②
End If
c = _________________________'③
s = s + c
Next a
List1.AddItem s
End Sub
(3)为了减缓字符串的出现速度,可将Timer1的Interval属性值________(选填,填字母: