I have got a little problem with setting a background image for `<button>`.

Here is the html I have got on site:

```
 <button id="rock" onClick="choose(1)">Rock</button>

```

And here is the CSS:

```
button {
   font-size: 18px;
   border: 2px solid #AD235E;
   border-radius: 100px;
   width: 150px;
   height: 150px;
}

button #rock {
   background: url(img/rock.png) no-repeat;
}

```

I don't know why the button's background is still white.