泛指的四种形式：
a+[c]sl.(可数名词的单数形式) 强调每一个
the+[c]sl.
[c]pl
[u]
例子： A bike is a very useful transport tool.
The bike is a very useful transport tool.
Bikes are very useful transport tool.
Knowledge is power(knowledge是不可数名词)
都表示自行车（泛指概念）是一种有用的交通工具.
特指的两种形式：
the+[c]pl.(可数名词的复数形式)
the+[u](不可数名词)
另外 名词后面有介词短语（I like the view from the window）分词短语（The milk sold in the shop is not fresh）定语从句（The girl who lives next door is calld Rose）,这些名词都是被特别修饰了的,表特指,应此应有the
（1）knowledge 在看到“a （good）knowledge of”的结构,有of必有a.其他情况下一般视为不可数名词,表泛指不用冠词.
（2）此处books 表泛指
（3）介词by表“以…计”,by the+[c]sl.：by the week/by the ton/by the yard/by the meter；by+抽象名词by volume/by weight/by length
例子：In America eggs are sold by the dozen.鸡蛋在美国是按打买的.
I'm paid by the month.我是按月计酬的.
= =我觉得应该是……while the food is sold by (the) pound食物是按磅卖的 是不是答案错了= =
（4）“这个价格”表特指
（5）in public是固定搭配
= =同学 拿一本语法书 仔仔细细认认真真安安心心读一遍记一遍 你的英语会有一个质的飞跃