Firdaus, M., Z. Wahab, M. Widiyanti, and M. S. Shihab. “Effect of Price and Promotion on Customer Loyalty of Domestic and Export Coal in the Time of the COVID-19 Pandemic”. International Journal of Social Sciences, vol. 5, no. 1, Jan. 2022, pp. 8-14, doi:10.21744/ijss.v5n1.1820.