逗号,因为that后面所引导的全部内容都要作为整个句子的真实主语.当然,如果that后面要表达几重意思,也可以用分号,就是不能用句号.
例如,
It can been seen that he loves her.大家都能看出他爱着她.
又如,
It can been seen that this world is still unrest,that peace is still desperately needed and that war should be avoided.
大家都能看到,世界仍不太平,我们仍极度需要和平,我们仍需要避免战争.