原始社会结束的根本原因是 [ ]
A．社会生产力的发展
B．私有财产的出现
C．禹转变为奴隶制国家的国王
D．阶级的产生
答案：A
While we were running on the playground, Jack suddenly stopped ________ and lay on the ground,
so we all stopped ________ what was wrong with him. [ ]
A. to run; to see
B. running; seeing
C. running; to see
( )是采用扩散处理方法，将一种或几种元素，从金属的表面扩散到基体金属中去使之形成扩散镀层。
A．热镀
B．渗镀
C．电镀
D．喷镀
