【B】
都可以表示原因.
A:because是直接因果关系,也就是说前面的事情直接导致了后面的结果.这里显然不是.
B:since表达的“原因”指的是一种“客观事实”,根据前面这样的事实,而影响的后面的结果.可以理解成“鉴于”“既然”,相当于"seeing that","now that"
Since you're not interested,I won't tell you about it.
C.as一般是给出后面主句结论的条件,也就是前面的事情推出后面的事情.
giving the reason for something:
比如：As you weren't there I left a message.因为你不在那里,我留了个信儿.
As she's been ill perhaps she'll need some help.她由於生病可能需要些帮助.
D For如果做连接词表示原因,是可以,但是不可以用于句首.