题干
— Lucy, what happened to Jim?
— A dog him the back.
A：bit; in
B：bite; on
C：bit; on
D：bites; in
答案(点此获取答案解析）
C
— Lucy, what happened to Jim?
— A dog him the back.
A：bit; in
B：bite; on
C：bit; on
D：bites; in
为等差数列，
是其前n项和，且
，则
的值为（ ）