题干
It was not until all the milu deer ________ that we realized we should build a natural park to protect them.
A：died off
B：died away
C：died out
D：died dow
答案(点此获取答案解析）
C
的前n项和，已知
，
，则
等于
，
，若以
、
为边作三角形，则
的取值范围（ ）
细胞分化的叙述错误的是