Help hiding rows based on drop down selection

One thing that didn't come to mind originally.
Going back to the dummy workbook, how could I add a piece of code that would allow two changes to come from one case.

IE:

If Target.Value = "a" Then
            Rows("18:30").Hidden = True
        Else
            Rows("18:30").Hidden = False
        End If

And, it would also hide Row 12.