This is an open access article distributed under the terms of the Creative Commons attribution License (http:// creative commons. org/ licenses/ by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
CC BY协议是所有CC协议中条件最为宽松的;CC BY 4.0是最新版本,也是OA期刊普遍采用的版本。