荀子说：“不闻不若闻之，闻之不若见之，见之不若知之，知之不若行之。学至于行而止矣。明之，行也。”材料中的“行”体现了（    ）

A．对事情要有耳闻

B．对事情要亲眼见之

C．要认识事物的本质和规律

D．要亲身实践

They are said () each other long time before they met in Harvard University where they worked together on a research project.

A. to know

B. to have known

C. knowing

D. having known