正是（）才使得大量磨制锋利的生产工具逐渐演变为互相残杀的武器，使用兵器的技艺及战争所需的格斗技术也逐步从生产技术中分离出来。
参考答案：人与人的搏杀格斗
1989年世界卫生组织确认，长期或大量摄入铝元素对人体神经系统将造成损害，建议限制导致人体摄入铝元素的各种应用。根据你的经验，举出三例受限制的应用。
Liu Fang ______ pipa since she was six years old. [ ]
A. played
B. was playing
C. had been playing
D. has been playing
