lab8.2 async serial exec with util modul

Hello,

In the lab 8.2 we have to run serialy (i am not sure we say like it, means serial execution) 3 async func.
I guess we should use "Promisify" as the util module is already import in the question.
But i don't get how to use it.

i can run the code without promisify

opA((err, d) => {
        print(err, d)
        opB((e, d) => {
                print(e, d)
                opC((e,d) => {
                        print(e, d)
                })  
        })  
})

i can run it with promise

function read(x) {
        return new Promise((resolve, reject) => {
                x((e,d) => {
                        if(e) reject(print(e))          
                        resolve(print(null, d)) 
                })
        })
}
async function as() {
        await read(opA)
        await read(opB)
        await read(opC)
}
as()

But i don t understand how to run it with Promisify ???
i can promisify,

const a = promisify(opA)
const b = promisify(opB)
const c = promisify(opC)

But it does not work inside async/await, and it does not resolve, so i can not use .then().
So what am i missing, what is the point of promisify here ??

Using it with function returning promise like readFile(), i understand how it works. But in this case, using it with call-back, i don t.

Thanks if someone can explain.

Regards

ps; the code to run

const { promisify } = require('util')

const print = (err, contents) => {
  if (err) console.error(err)
  else console.log(contents)
}

const opA = (cb) => {
  setTimeout(() => {
    cb(null, 'A')
  }, 500)
}

const opB = (cb) => {
  setTimeout(() => {
    cb(null, 'B')
  }, 250)
}

const opC = (cb) => {
  setTimeout(() => {
    cb(null, 'C')
  }, 125)
}