题干
This problem is said _____ three times.
A：having discussed
B：to have been discussed
C：having been discussed
D：to have discussed
答案(点此获取答案解析）
B
在（
，+
）内有定义．对于给定的正数K，定义函数
，取函数
=
．若对任意的
，恒有
=