Question

___all the students are here, why don't we start at once?

Answer 1

你好，我觉得应该选择C
since的用法主要有两点：一是表示“自从……以来”，二是表示“既然”之意
在此句中就是“既然”的意思。强调已知的事实
既然所有的学生都在这儿了，我们为什么不立刻出发呢？
例：Since
you
can't
answer
the
question,
I'll
ask
someone
else.
既然你回答不出这个问题，我就问别人了。
As
soon
as
意思是“一……就……”
I
will
tell
him
as
soon
as
I
see
him.我一看到他就会告诉他。
I'll
write
you
as
soon
as
I
get
there.
我一到那儿，就给你写信。
when
从句表示某时刻或一段时间
例如：Sorry，I
was
out
when
you
called
me．
对不起，你打电话时我刚好外出了。