How get vars if I use LoadVars.onData

programming flash
1

I use this code and try to get data from simply txt file.
I explored with onData method.
AS:

var myLV:LoadVars = new LoadVars ();
myLV.onData = function (scr:String)
{
	if (scr == undefined)
	{
		trace ("Error loading content");
	}
	else
	{
		_level0.createTextField ("scr_txt", 1, 10, 10, 200, 200);
		_level0.scr_txt.border = true;
		_level0.scr_txt.text = scr;
		_level0.createTextField ("a_txt", 2, 10, 300, 200, 200);
		_level0.a_txt.border = true;
		_level0.a_txt.text = myLV.var_a;
	}
};
myLV.load ("myText.txt", myLV, "GET");

How can I get the value of var_a?
Shoud I use for it method onload ?

Thanks
TXT file has those vars:

var_a=Hello&var_b=World