potatoes are a vegetable the potatoes are a vegetable a potato is a vegetable

potatoes are a vegetable the potatoes are a vegetable a potato is a vegetable

The potato is a vegetable.是比较恰当的表述
vegetable表示“蔬菜”,是可数名词,但通常用复数形式.
例如We grow a lot of different vegetables.
但是此处表述蔬菜,代表一种属性,所以要用单数.