cause signal words

because / as / since
I do not support animal testing. It is inhuman.
I do not support animal testing because it is inhuman.

这里有个纠结： because 前要不要加逗号，答案是否定的，绝对不能加逗号。同样的 as, since 前面也不能加逗号。

但是表原因的 For 前是可以加逗号。

原因：
for 对等结构 =》 需要加逗号
because, as, since 是从属结构 =》 不需要加逗号。
如果原因在前，逗号不可少
Because it is inhuman, i do not support animal testing.