问题补充：
I know the teacher Mary when he spoke of a bright girl.
A. referred to
B. had referred to
C. was referring to
D. refers to
答案：
A. referred to
B. had referred to
(答案→)C. was referring to
D. refers to
解析：考查时态。句意“我知道那个老师当说到一个聪明的女孩时，他说的是玛丽。”根据when从句可知，说的同时，伴随的动作，应该用过去进行时。refer to“提到，指”。故选C。
