单选题 Our boss was surprised to hear that we'd finished our work so ______.
A. quickest B. quicken
C. quickly D. quicker

【正确答案】 C
【答案解析】[解析] 空格是修饰work并且能够用副词so修饰的位置，所以quickly最合适。D项quicker这样的比较级不能够用so修饰。