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li a {	background-color:yellow;  	 display: inline-block;     text-align: center;     padding: 14px 16px;     text-decoration: none; } 
 li a:hover {  background-color: red; } 
 nav ul {  border:0px solid red;      list-style-type: none;     margin: 0;     padding: 0;  }  
nav ul li {  float: left;     }  
nav ul li ul {  display: none; }  
nav ul li:hover > ul { display: block;  	}

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 ...
U2
     

       
U2a
       
U2b
     
   
    
U3
...