For Android version, If you encounter download or installation problem, please refer to:
1.  While installing the Etutor.iClass app, if screen pop-up For your security, please tap Settings.
当安装 Etutor.iClass app 时,屏幕提示 For your security,点按 Settings 。
2.  Swich on Allow from this source.
打开 Allow from this source。
3.  Back to Install unknown apps.
返回app 安装界面
4.  Install App.