怎么编写交通红绿灯的程序（VB）考试求救啊！

十字路口红绿灯程序，红灯倒计时30秒，绿灯倒计时30秒，
黄灯倒计时5秒
Dim A As Integer, B As Integer, C As Integer

Private Sub Form_Load()
'启动
Timer1.Enabled = True
Timer2.Enabled = False
Timer1.Interval = 1000
Timer2.Interval = 1000
A = 31
B = 31
C = 5
End Sub

Private Sub Timer1_Timer()
'红灯时间绿灯时间
A = A - 1
B = B - 1
Label3.Caption = ""
If A = 0 And B = 0 Then
Timer1.Enabled = False
Timer2.Enabled = True
C = 6
End If
Label1.Caption = "红灯时间=" & Str(A) & "秒"
Label2.Caption = "绿灯时间=" & Str(B) & "秒"
End Sub
Private Sub Timer2_Timer()
'黄灯时间
Label1.Caption = ""
Label2.Caption = ""
C = C - 1
If C = 0 Then
Timer1.Enabled = True
Timer2.Enabled = False
A = 31
B = 31
End If
Label3.Caption = "黄灯时间=" & Str(C) & "秒"
End Sub

详细详细。