knife是可数名词还是不可数名词?

　　Score the card first with a knife.

　　先用刀在卡片上划出痕迹。

　　The deliberate cruelty of his words cut her like a knife.

　　他故意说的残酷无情的话对她像刀割一样。

　　Mind you don't cut yourself ─ that knife's very sharp.

　　小心别伤着——这刀子快得很。