AJAX :: CascadingDropDown - Method Error 500 Only For One DropDown And Service Path Is Correct

I have 3 table in database

1-state

Id state
1 USA
2 UK

2-City

Id name state
1 Canada USA
2 London UK

3-Region

Id regionname cityid
1 01 Canada
2 02 London

and I have 3 DropDownList that refer below thread [URL] ...

I use cascadingdropdown for these 3 dropdownlist below are codes:

AND

[WebMethod]
public CascadingDropDownNameValue[] GetCountries(string knownCategoryValues)
{
string query = "SELECT state FROM stateTest";
List countries = GetData(query);
return countries.ToArray();
}

[Code] ....

1-DDLstate

2-DDLcity

3-DDLregion

at first I should select Item from DDLsate then DDlcity and at the end I should select Item from DDLregion

above code work correctly for ddlstate and ddlcity but it doesn't work for ddlregion...

I mean when I select Item from ddlcity in ddlregion shows:[Metod error500]