Which of these is a structural isomer of butanol?

A.
```
H H H H
| | | |
H-C-C-C-C-H
| | | |
H H H H
```

B.
```
H H H
| | |
H-C-C-C-H
| | |
H H H
|
OH
```

C.
```
H H H
| | |
H-C-O-C-H
| | |
H H H
```