Through years of hard work, Mrs. Smith, who ____ as a secretary for five years, is general manager of the company now.

A．serves

B．served

C．has served

D．had served

下列属于通过热传递改变物体内能的实例是（　　）

A．人类最早的取火方式--“钻木取火”

B．电热毯中的电阻丝通电后温度升高

C．冬天，用热水袋取暖

D．巨大的陨石堕入大气层后变成流星