combobox怎么用

'建combo1和combo2，combo1下拉里有a、b两个，
'combo2下拉有1、2、3、4四个。当combo1选a时，
'combo2里只有1、2俩个选项，当combo1选b时，combo2里只有3、4俩个选项。
'请教各位高手，程序该怎么写啊

Private Sub Combo1_click()
Select Case Combo1.ListIndex
Case 0
Combo2.Clear
Combo2.AddItem "1"
Combo2.AddItem "2"
Case 1
Combo2.Clear
Combo2.AddItem "3"
Combo2.AddItem "4"
End Select
Combo2.Text = "请选择..."
Combo2.Enabled = True
End Sub

Private Sub Form_Load()
Combo1.AddItem "a"
Combo1.AddItem "b"
Combo2.Enabled = False
Combo1.Text = "请选择..."
End Sub

'窗体上放两个combobox控件

Private Sub Combo1_click()
Combo2.Clear
If Combo1 = "a" Then Combo2.AddItem "1": Combo2.AddItem "2"
If Combo1 = &q