Funding for this research was provided by:
China Postdoctoral Science Foundation funded project (2017M612231)
the Project of Shandong Province Higher Educational Science and Technology Program (J14LI08)
Doctoral Scientific Research Foundation of Qufu Normal University and Youth Foundation of Qufu Normal University (BSQD20130140)
Youth Foundation of Qufu Normal University (XKJ201303)
Article History
Received: 3 January 2018
Accepted: 2 May 2018
First Online: 8 May 2018
Competing interests
: The authors declare that they have no competing interests.