—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：考查定语从句。该定语从句的先行词是a Saturday，关系词在从句中作时间状语，所以用关系副词when。句意是; --你哥哥是什么时候开始显示出他对医学的兴趣。--是在一个星期天，当时他看他同学给青蛙做实验。选C。该题考生常误选that，认为是强调句型。
考点:考查定语从句
点评;解答有关定语从句题目的关键是：先找出先行词，然后把先行词代入从句中，判断其在从句中的地位和作用。