(2013·重庆模拟)—When did your brother begin to show his interest in medicine?
—It was a Saturday he noticed his classmate experimenting on a frog in the lab.
A. that
B. what
C. when
D. which
网友回答
A. that
B. what
(答案→)C. when
D. which
解析：选C。考查定语从句。结合句意并分析句子结构可知, 此处使用when引导一个定语从句, when在从句中作状语。