为什么当鼠标悬停在图像上时，文本不显示？如何在css中使用visibility属性获得文本的可见性

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img{
  margin-top: 110px;
  display: block;
  margin-left: auto;
  margin-right: auto;
  width: 50%;
  margin-top: 50px;
  position: relative;
}



.txt{
  font-size: 16px;
  line-height: 20px;
  margin-top: 10px;
  text-align: center;
  margin-left: 440px;
  margin-right: 440px;
  visibility: hidden;
  
}

img:hover  .txt{
  cursor: pointer;
  visibility: visible;
}

 <div class="container"> 
<img src="empty-boxes.svg"  style="width:520px;height:190px;">
<p class="number">404</p>
<p class="txt">When communicating via HTTP, the server is required
  .</p>
  </div>

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