The bird were ① enough .They really knew ② the cat was .⑴A .

The bird were clever enough.They really knew where the cat was.
鸟儿们足够聪明.他们的确知道猫在哪里.
如果你用who,意思是猫是谁.其实也不是不可以.假如你是在写童话,里面有个猫先生叫做Tom.是个非常厉害的侦探.它最近在查一起首饰被偷的案子.它怀疑小鸟Judy偷了.于是开始蹲点.而鸟Jerry的全家都给它打掩护.

不能，who是指代人的，不用在猫身上，即便那猫长得像人

不能的。