knife是可数名词还是不可数名词

例句：

Bread knives should have a serrated edge.

面包刀的`刀刃应带有波浪齿。

Two knife racks hold her favourite knives.

两个刀架上放着她最喜爱的刀。

2 000 knives have been handed in during the month-long amnesty.

在一个月的赦免期中交出了2 000把刀。