Use variable for json answer in QML BlackBerry

Question

I have a project in QML, MomenticsID (BlackBerry10). I need to use the variable for json answer. I can not find some manual for this problem. When I use var result = data.item_01[0] everything is OK and program results in correct value. But when I use var result = data.variable I won't get an answer.

main.qml

import bb.cascades 1.4
import bb.data 1.0

Page {
    id: page

    property string variable: "item_01[0]"

    Container {

        Label {
            id: label
        }
    }

    attachedObjects: [

        DataSource {
            id: dataSource
            type: DataSourceType.Json
            source: "asset:///data.json"

            onDataLoaded: {
                var result = data.item_01[0] // it's OK, result is "a"
                //var result = data.variable // it's NOK, nothing result

                label.text = result.answer_01
            }
        }
    ]

    onCreationCompleted: {
        dataSource.load()
    }
}

data.json

{
  "item_01": [
    {
      "answer_01": "a"
    }
  ]
}