The science Festival is an occasion model planes and model rockets are flown over the school grounds.
A. that
B. when
C. where
D. which
网友回答
A. that
(答案→)B. when
C. where
D. which
解析：考查定语从句：句意：科学节是模型飞机和模型火箭飞越校园的机会。先行词是occasion，机会，场合，表示“时间”，定语从句中不缺主语，宾语和表语，缺少的是时间状语，用when引导定语从句，选B。