css,DIV框内块元素增加,这个DIV的框高(height)也随着增加。这个怎么写?#big{width:100px; height:300px; background:red;}#sa{width:100px; height:280px; background:green;}<div id="big"> <div id="sa">1</div> <div id="sa">2</div> <div id="sa">3</div></div>