28. Up to Tuesday’s attack, the worst school shooting in the U.S. was in March 1998 in Jonesbora, _____ two boys, aged 11 and 13, shot and killed a teacher and four girls.

A. while　 　　　　　 B. where　　 　　　　　　　 C. when　 　　　　　　　　　 D. then