div居中怎么弄

<style>
.a{border:1px solid red;height:50px;}
.b{border:1px solid blue;float:left;}
.c{border:1px solid green;float:left;}
</style>
<div class="a">
<div class="b">bbbbbbbbbbbbbbbbb</div>
<div class="c">ccccccccccccccccc</div>
</div>

我想让b和c 两个div 居中，该怎么弄
[解决办法]

<style>
    .a{border:1px solid red;height:50px;}
    .b{padding-left:12px;padding-right:12px;border:1px solid blue;}
    .c{border:1px solid green;float:left;}
    .d{width:300px;margin:0 auto;}
    .b,.c{text-align:center;}
</style>
<div class="a">
<div class="d">
    <div class="b">bbbbbbbbbbbbbbbbb</div>
    <div class="c">ccccccccccccccccc</div>
</div>
</div>