I have the following list of data frames:

```
l = list()
l[[1]] = list(A=iris[1:3,], B=mtcars[1:3,])
l[[2]] = list(A=iris[5:8,], B=mtcars[5:8,])

```

What I want to do is to concatenate only `A` element of the list; resulting in:

```
Sepal.Length Sepal.Width Petal.Length Petal.Width Species
1          5.1         3.5          1.4         0.2  setosa
2          4.9         3.0          1.4         0.2  setosa
3          4.7         3.2          1.3         0.2  setosa
5          5.0         3.6          1.4         0.2  setosa
6          5.4         3.9          1.7         0.4  setosa
7          4.6         3.4          1.4         0.3  setosa
8          5.0         3.4          1.5         0.2  setosa

```

I tried this but it doesn't give what I want:

```
> do.call(rbind, l)
     A      B      
[1,] List,5 List,11
[2,] List,5 List,11

```

What's the right way to do it?