Using these dataset:

```
[A]
100
200
300

[B]
A
B
C

```

I would like to make this column:

```
[A]   [B]
100    A
100    B
100    C
200    A
200    B
200    C
300    A
300    B
300    C

```

I would like to use rep function in R, but it does not work.
How do I create this column?