Javascript: Form problem

Hello,

I got many Select Boxes in a HTML form, and they all get a different select box name. Eg.

<SELECT Name="Hello_100" OnChange="MyTest(6, 'Hello_100', this.form)">
<OPTION Value="1">Read</OPTION>
<OPTION Value="2">Write</OPTION>
<OPTION Value="3">Delete</OPTION>
</SELECT>

<SELECT Name="Hello_154" OnChange="MyTest(2, 'Hello_154', this.form)">
<OPTION Value="1">Read</OPTION>
<OPTION Value="2">Write</OPTION>
<OPTION Value="3">Delete</OPTION>
</SELECT>

<SELECT Name="Hello_291"OnChange="MyTest(3, 'Hello_291', this.form)">
<OPTION Value="1">Read</OPTION>
<OPTION Value="2">Write</OPTION>
<OPTION Value="3">Delete</OPTION>
</SELECT>

and I got a javascript function:
function MyTest(ID, selectboxname, theform)
{
// how do I get the value of the selected option?
alert(thisform.selectname.options[thisform.selectname.selectedIndex].value);
}


As you can see in my javascript function MyTest, the statement
thisform.selectname.options[thisform.selectname.selectedIndex].value
will not return the selected option value.
How do I get the selected option value? The selected box name is generated randomly.
pls help.

<SELECT Name="Hello_100" OnChange="MyTest(6, this.name, this.value)">
<OPTION Value="1">Read</OPTION>
<OPTION Value="2">Write</OPTION>
<OPTION Value="3">Delete</OPTION>
</SELECT>

function MyTest(ID, selectboxname, thevalue)
{
alert(thevalue);
}