It's no longer a question _____ man can go to the moon.

that.that 引导同位语从句,用来说明的qestion具体内容.问题是什么? 问题是从句所表示的内容.
For example:
There is no doubt that you will succeed in time.

that 这里的man是泛指人类,就是说人类登上月球不是问题了
that是question的从句~

从句,需要一个引导词:THAT.

它不再是_____人能去月亮的一个问题。

that

that