问题
选择题
(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
答案
答案：C
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。