Received: 6 January 2019
Accepted: 28 March 2019
First Online: 9 April 2019
Ethics approval and consent to participate
: The survey was approved by the Ethics Committee of the University of Toyama (H28-137). Written informed consent was obtained from all the school children and their parents, and participation was voluntary.
: Not applicable.
: The authors declare that they have no competing interests.
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