I have two lists as follow: 

```
l={a,b,c,d}

```

and

```
ll={{1,x},{2,x},{3,x},{4,x}}

```

I want to replace the x's with a,b,c and d so that the outcome would be: 

```
{{1,a},{2,b},{3,c},{4,d}}

```

How would one does this?