Qiqing Mo, a visiting graduate student at University of Florida, coming from Guangxi Medical University, participated in data collection. This study would take longer time to complete without her assistance.

Authors’ contributions
Chen X and Yu B conceived the topic together and joined conducted the analysis and participated in manuscript development. Yu B spent time to collect data, verify results and review and comments on the manuscript first drafted by Chen X. The author(s) read and approved the final manuscript.

Funding
None.

Ethics approval and consent to participate
Not applicable.

Consent for publication
Not applicable.

Competing interests
The authors declare that they have no competing interests.