怎么设置css字体单行居中

<style>
    .app{
        width: 200px;
        height: 100px;
        border: 1px solid skyblue;
    }
</style>
<div class="app">
    
</div>

<style>
    .app{
        width: 200px;
        height: 100px;
        border: 1px solid skyblue;
    }
    .app > p{
        text-align: center;
        overflow: hidden;　
        text-overflow:ellipsis;　　
        white-space: nowrap;
    }
</style>
<div class="app">
    <p>Hello World！</p>
</div>

<style>
    .app{
        width: 200px;
        height: 100px;
        text-align: center;
        border: 1px solid skyblue;
    }
</style>
<div class="app">
    <span>Hello World！</span>
</div>

<style>
    .app{
        width: 200px;
        height: 100px;
        text-align: center;
        border: 1px solid skyblue;
    }
    .app > span{
        line-height: 100px;
    }
</style>
<div class="app">
    <span>Hello World！</span>
</div>