onchange event for dropdown list

Question

var DropdownList = document.getElementById('<%=ACan.ClientID %>');

var SelectedIndex = DropdownList.SelectedIndex;---- (if debug the code thn here value is not defined)

var divtenper = document.getElementById('divtenper');

var divyear10 = document.getElementById('divyear10');

var divpertwelth = document.getElementById('divpertwelth');

var divyearof12 = document.getElementById('divyearof12');

var divgraper = document.getElementById('divgraper');

var divyearofgra = document.getElementById('divyearofgra');

//divtenper.style.display = Course.value == "1" ? "block" : "none";

if (SelectedIndex == 1) {

divpertwelth.style.display = "none";

divyearof12.style.display = "none";

divgraper.style.display = "none";

divyearofgra.style.display = "none";

divyear10.style.display = "block";

divtenper.style.display = "block";

}

else if (SelectedIndex == 2) {

divpertwelth.style.display = "block";

divyearof12.style.display = "block";

divyear10.style.display = "none";

divtenper.style.display = "none";

divgraper.style.display = "none";

divyearofgra.style.display = "none";

}

else if (SelectedIndex == 3) {

divgraper.style.display = "block";

divyearofgra.style.display = "block";

divpertwelth.style.display = "none";

divyearof12.style.display = "none";

divyear10.style.display = "none";

divtenper.style.display = "none";

}

// return true;

}

</script>

 

then not show my div