Android : Coverting json array using retrofit?

this is my Json

[  
{  
  "nata_center":{  
     "id":67,
     "nata_center_name":"Primo Institute of Design"
  }
 },
{  
  "nata_center":{  
     "id":68,
     "nata_center_name":"Sai Ganesh Institute"
  }
 }
]

Pojo classes

public class Explorer {
NataCenter nataCenter;

public NataCenter getNataCenter() {
    return nataCenter;
}

public void setNataCenter(NataCenter nataCenter) {
    this.nataCenter = nataCenter;
}
}

2)

public class NataCenter {
public String id;
public String nata_center_name;

 public NataCenter(String id,String nata_center_name)
 {
  this.id=id;
 this.nata_center_name=nata_center_name;
 }

public void setId(String id) {
    this.id = id;
}

public String getId() {
    return id;
}

public String getNata_center_name() {
    return nata_center_name;
}

public void setNata_center_name(String nata_center_name) {
    this.nata_center_name = nata_center_name;
}

}

GetMethhodinRetrofit

void getCenter(@Query("id") int id,Callback<List<Explorer>> callback);

MainActvitiy.java

       service.getCenter(i,new Callback<List<Explorer>>() {
                        @Override
                        public void success(List<Explorer> o, Response response) {
                            Log.d(TAG,"Success" + "Response"+o.toString());

                        @Override
                        public void failure(RetrofitError error) {
                            Log.d(TAG,"Failed"+error.toString());

                        }
                    });

This is the error message

 Failedretrofit.RetrofitError: com.google.gson.JsonSyntaxException: java.lang.IllegalStateException: Expected BEGIN_ARRAY but was BEGIN_OBJECT at line 1 column 2 path $