The Oxford · Keys Primary Supplementary Exercises “Regular Training Series” is of moderate difficulty, suitable for students whose test performance is relatively average. It helps them strengthen their foundation, clarify abstract concepts, improve grammar, and expand vocabulary, ensuring comprehensive mastery of topics taught in school. This series aims to turn challenges into opportunities and effectively boost test scores. Each book focuses on different learning objectives, and once students complete the entire “Regular Training Series,” they can advance to the “Elite Enhancement Series”!