Deep understanding of async await asynchronous programming synchronization

we know that the JavaScript code executes sequentially, so let’s look at the result of the execution of the following code: 

function a(){
    return new Promise((resolve,reject)=>{
        resolve("a")
    })
}
function b(){
    return new Promise((resolve,reject)=>{
      setTimeout(() => {
        resolve("b")
      }, 2000);
    })
}
function c(){
    return new Promise((resolve,reject)=>{
        resolve("c")
    })
}
 function execute(){
    let a1= a()
    a1.then(res=>{
        console.log(res);
    })
    let b1= b()
   b1.then(res=>{
    console.log(res);
   })
    let c1= c()
   c1.then(res=>{
    console.log(res);
   })
}
execute()


so how to show it in order, that is, C wait for B to execute after the execution. The following 

function a(){
    return new Promise((resolve,reject)=>{
        resolve("a")
    })
}
function b(){
    return new Promise((resolve,reject)=>{
      setTimeout(() => {
        resolve("b")
      }, 2000);
    })
}
function c(){
    return new Promise((resolve,reject)=>{
        resolve("c")
    })
}
async function execute(){
    let a1=await a()
    //用了await之后，就不用.then了直接得到结果
   console.log(a1)
    let b1=await b()
    console.log(b1)
    let c1=await c()
   console.log(c1)
}
execute()

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