An apple a day keeps (the doctor)/(doctors) away?

问题描述:

An apple a day keeps (the doctor)/(doctors) away?
An apple a day keeps the doctor away?
为什么不是
An apple a day keeps doctors away?
不是用名词复数表示一类事物吗?

the doctor 表示医生这类人
此句的意思主要体现每天一个苹果没有毛病,关键不在医生,所以不用doctors