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.