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
网友回答
A. while
B. where
(答案→)C. when
D. 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。