In some cultures ______ that a guest should stay for no more than three days.

In some cultures ______ that a guest should stay for no more than three days.
A thinking B to think
C it thinksD it is thought
请问这道英语单选选哪个呢?哪位老师达人知道希望告诉我一下.
请告知为什么选那项,别的为什么不选?最后还请翻译一下这句话.多谢多谢~~~.