题干
—I feel like ________for a walk. Will you go with me?
— If Mary goes, __________.
A：to go; I also
B：to go; so do I
C：going; so will I
D：going; so do I
答案(点此获取答案解析）
C
}为等差数列，公差d = -2，
为其前n项和.若
，则
=（ ）