—When did your cousin begin to show his interest in medicine?—It was a Saturday she notic

问题补充：

—When did your cousin begin to show his interest in medicine?

—It was a Saturday she noticed her uncle experimenting on a frog in his lab.

A. that

B. what

C. when

D. which

答案：

A. that

B. what

(答案→)C. when

D. which

解析：本题修饰名词Saturday为定语从句，先行词Saturday为时间，关系词应该用when，故本题C。