SARS is an illness that can result in death unless left in time.A. to treatB. treatingC.

问题补充：

SARS is an illness that can result in death unless left in time.

A. to treat

B. treating

C. treated

D. treat

答案：

A. to treat

B. treating

(答案→)C. treated

D. treat

解析：本句中的treat 充当主语补足语，且与主语SARS之间为被动关系，故应选择过去分词。