题干
This is ______ eraser. That is ______ ruler.
A：an; an
B：a; a
C：an; a
D：a; an
答案(点此获取答案解析）
C
This is ______ eraser. That is ______ ruler.
A：an; an
B：a; a
C：an; a
D：a; an
的导函数是
，则数列
的前
项和为（ ）
