function fillCities(optie)
{ 
   var selbox = document.getElementById('plaats_possible');
	 selbox.options.length = 0;

   if (optie == '6') 
   { 
		 selbox.options[selbox.options.length] = new Option('Brakel (Gelderland)','2306');	 }
   if (optie == '9') 
   { 
		 selbox.options[selbox.options.length] = new Option('Gorinchem (Zuid-Holland)','3891');	 }
   if (optie == '11') 
   { 
		 selbox.options[selbox.options.length] = new Option('Babyloniënbroek (Noord-Brabant)','4706');		 selbox.options[selbox.options.length] = new Option('Drongelen (Noord-Brabant)','4963');		 selbox.options[selbox.options.length] = new Option('Dussen (Noord-Brabant)','4972');		 selbox.options[selbox.options.length] = new Option('Eethen (Noord-Brabant)','4978');		 selbox.options[selbox.options.length] = new Option('Genderen (Noord-Brabant)','5030');		 selbox.options[selbox.options.length] = new Option('Hank (Noord-Brabant)','5082');		 selbox.options[selbox.options.length] = new Option('Hedikhuizen (Noord-Brabant)','5094');		 selbox.options[selbox.options.length] = new Option('Kaatsheuvel (Noord-Brabant)','5266');		 selbox.options[selbox.options.length] = new Option('Nieuwendijk (Noord-Brabant)','5528');		 selbox.options[selbox.options.length] = new Option('Waalwijk (Noord-Brabant)','5895');	 }
   if (optie == '12') 
   { 
		 selbox.options[selbox.options.length] = new Option('Maastricht (Limburg)','6443');} 
}