Hambali, Muslih, Soni Mirizon, and Nandang Heryana. “Difficulty Index and Cognitive Skills of English Textbook for Senior High School”. Indonesian Journal of EFL and Linguistics 6, no. 1 (May 19, 2021): 17-28. Accessed February 10, 2026. https://www.indonesian-efl-journal.org/index.php/ijefll/article/view/331.