从A、B、C、D四个选项中，选出可以入空白处的最佳选项 6. It is often that human beings are natur

从A、B、C、D四个选项中，选出可以入空白处的最佳选项

6. It is often that human beings are naturally equipped to speak.

A. said B. to say C. saying D. being said

请帮忙给出正确答案和分析，谢谢！