Every Friday is the school open day , parents can go to the class to observe it .
A. where
B. when
C. whose
D. which
网友回答
A. where
(答案→)B. when
C. whose
D. which
解析： 考查定语从句。先行词是the school open day ,在定从 parents can go to the class to observe it中作时间状语，所以选B。