option1
    option2
    option3
    option4



 function showHide()
 {
   hideAll();
  var val = document.getElementById("dropdown").value;

  if(val == "A")
   document.getElementById("firstTextBoxId").style.display = 'block';
  else if(val == "B")
   document.getElementById("secondTextBoxId").style.display = 'block';
  else if(val == "C")
   document.getElementById("ThirdTextBoxId").style.display = 'block';
  else if(val == "D")
   document.getElementById("FourthTextBoxId").style.display = 'block';

}


function hideAll()
   {
      document.getElementById("firstTextBoxId").style.display = 'none';
      document.getElementById("secondTextBoxId").style.display = 'none';
      document.getElementById("thirdTextBoxId").style.display = 'none';
      document.getElementById("fourthTextBoxId").style.display = 'none';

    }