Which missing line of code would place the text on top of the image?

<div class="container">
  <img src="grumpy-cat.gif" />
  <p>The z-index property is cool!</p>
</div>

img {
  position: absolute;
  left: 0px;
  top: 0px;
  // Missing line
}

<div class="container">
  <img src="grumpy-cat.gif" />
  <p>The z-index property is cool!</p>
</div>

img {
  position: absolute;
  left: 0px;
  top: 0px;
  // Missing line
}

1.  z-index: 1;z-index: 1;
2.  z-index: -1;z-index: -1;
3.  z-index: 0;z-index: 0;
4.  z-index: true;z-index: true;