Received: 4 April 2019
Accepted: 5 July 2019
First Online: 13 July 2019
Ethics approval and consent to participate
: The consent was obtained by verbal from parent and the questionnaire was answered voluntarily. The reason of verbal was that our study was anonymous questionnaire and the individual could not be identified. Institutional review board approval for this study was granted by the Niigata City Board of Education.
: Not applicable.
: The authors declare that they have no competing interests.