哪个CSS片段通常用于水平居中?
A. site-align:center;
B. margin:center;
C. margin:auto0;
D. margin:0auto;
如何使用CSS3调整背景图像的大小?
A. background-size:80px60px;
B. bg-dimensions:80px60px;
C. background-proportion:80px60px;
D. alpha-effect:bg-resize80px60px;
如何清除浮动?
A. clear
B. overflow
C. margin
D. border