问题补充：
In the dark street there wasn’t a single person Mary could turn for help.
A. to whom
B. to which
C. on whom
D. on which
答案：
(答案→)A. to whom
B. to which
C. on whom
D. on which
解析：考察定语从句，该句为定语从句， 先行词是single person, 在从句中作turn to的宾语，turn to sb for help意为向某人求助，to 是介词，后面指人的时候只能选择whom，所以选择A
