Nguyen, T. T. H., and D. Jongkonklang. “A Study of 21st Century Learning Skills for High School Students in Cao Bang Province, Vietnam”. KKU Research Journal (Graduate Studies) Humanities and Social Sciences, vol. 6, no. 2, June 2018, pp. 14-24, https://so04.tci-thaijo.org/index.php/gskkuhs/article/view/130976.