合并句子(我自己做的并且老师已讲评）willpeopleliveonMarsbytheyear2100?Areyousure...AreyousureifpeoplewillliveonMarsbytheyear2100?为什么要加个if呢?而且看到另外一句句子我更费解了：Areyousu

Question

合并句子(我自己做的 并且老师已讲评）
will people live on Mars by the year 2100?Are you sure...
Are you sure if people will live on Mars by the year 2100?
为什么要加个if呢?而且看到另外一句句子我更费解了：
Are you sure they will arrive on time?
那个if到底是什么含义 而且我举得两个例子应该是属于同类型的 可是一个加了一个没加..顺便 请你们根据的回答.把下面一句话合并一下.
Did China send spacemen into outer space last year?Do you know...
The plan is under____(讨论）.We haven't decided it yet.
我的答案是discussing,但老师说是discussion.我不理解.我觉得至少不可能填一个名词啊
afraid的句型是不是有两个 ：1.be afraid of sth.2.be afraid to do sth.
目前只有这三个问题 请知道的务必告诉我

Answer 1

Are you sure that people will live on Mars by the year 2100?
Are you sure （that） they will arrive on time?
that可以省略 你们老师错了 sb is/are sure that 从句 表示 ...确信... 不能接 whether/if等引导的从句
Do you know whether/if China sent spacemen into outer space last year?你知道 表示不是确定的事 用 whether/if 表两种可能
under 介词 后面接名词
be afraid of/to do/that