<ul id="menu">
    <li>A</li>
    <li>B
        <ul class="submenu">
            <li>I</li>
            <li>II</li>
            <li>III</li>
            <li>IV</li>
        </ul>
    </li>
    <li>C</li>
</ul>

#menu>li {
    float: left;
    position: relative;
}
#menu>li .submenu {
    display: none;
    position: absolute;
    width: 324px;
}
#menu>li:hover .submenu {
    display: block;
}
.submenu li {
    padding-left: 5px;
    width: 75px;
    height: 24px;
    float: left;
}