Up to Tuesdays attack, the worst school shooting in the U. S. was in March 1998 in Jones

Up to Tuesday's attack, the worst school shooting in the U. S. was in March 1998 in Jonesboro, two boys, aged 11 and 13, shot and killed a teacher and four girls.
A. while
B. where
C. when
D. then

A. whileB. where(答案→)C. whenD. then解析：本题关键是辨明横线后为完整句“two boys，aged 11 and 13，shot and killed a teacher and four girls'’，其中aged 11 and 13为插入语。

我明天再问问老师，叫他解释下这个问题