JS selection

<script type="text/javascript">
var dropdownProduct = document.getElementById("input_137");
var paypalProduct1  = document.getElementById("input_116_1006");
var paypalProduct2  = document.getElementById("input_116_1005");
var paypalProduct3  = document.getElementById("input_116_1004");
// Disabling PayPal products selection
window.onload = function(){
  paypalProduct2.disabled = true;
  paypalProduct3.disabled = true;
};
// Corresponding selection from Dropdown to PayPal products
function makeSelection() {
  if (dropdownProduct.selectedIndex == 1) { // Registering one player option
    paypalProduct1.disabled = false;
    paypalProduct2.disabled = true;
    paypalProduct3.disabled = true;
    paypalProduct1.checked  = true;
  } else if (dropdownProduct.selectedIndex == 2) { //Registering two players option
    paypalProduct1.disabled = true;
    paypalProduct2.disabled = false;
    paypalProduct3.disabled = true;
    paypalProduct2.checked  = true;
  } else if (dropdownProduct.selectedIndex == 3) { //Registering three players option
    paypalProduct1.disabled = true;
    paypalProduct2.disabled = true;
    paypalProduct3.disabled = false;
    paypalProduct3.checked  = true;
  };
};
</script>