Private Sub Worksheet_Change(ByVal Target As Range)
Application.ScreenUpdating = False
If Target.Address <> "$D$17" Then Exit Sub
If Target > 2 Or Not IsNumeric(Target) Then Exit Sub: Rows.Hidden = 0
Rows(Choose((Target + 2) - 1, "21:37", "27:37", "33:37")).Hidden = -1
End Sub