let's have a _(drink)of orange答案是drink,为什么?

let's have a _(drink)of orange答案是drink,为什么?
let's have a _(drink)of orange
答案是drink,为什么?

let's have a _(drink)of orange答案是drink,为什么?
let's have a drink(drink)of orange .have a ___ 句式,动作名,如,have a look

drink在做名词的时候是可数名词。