set string field in an entity to retrieve data from option set of another entity via lookup

I have 2 custom entities, one entity (A) has 2 fields (new_subject (string)& new_agreementtemplate(lookup to entity B)) , another entity (B) has 2 fields (new_subject (option set)& ID).

I want user to select a record of lookup field(new_agreementtemplate) in "A" entity, subject fills via subject of lookup field. I wrote below code, But it doesn't work because of option set if I choose string filed It works correctly.

Would you please help me?

function DefaultSubjectValueOnOrder()
{

var tempID= Xrm.Page.getAttribute("new_agreementtemplate").getValue();

if (tempID!= null) {

var ID= Xrm.Page.getAttribute("new_agreementtemplate").getValue()[0].id
var entitytype= Xrm.Page.getAttribute("new_agreementtemplate").getValue()[0].entityType;

var url = Xrm.Page.context.getClientUrl() +"/XRMServices/2011/Organizationdata.svc/" + entitytype +
"Set(guid'" +ID + "')?$select=new_Subject" ;

var req = new XMLHttpRequest();
req.open("GET", url, false);
req.setRequestHeader("Accept", "application/json");
req.setRequestHeader("Content-Type", "application/json; charset=utf-8");
req.send();

var results = JSON.parse(req.responseText).d.new_Subject.value;

Xrm.Page.getAttribute("new_subject").setValue(results);

}

}