当元素浮动导致它的父元素.bigbox塌陷时，可以使用（ ）清除浮动 A: .bigbox::after{ display: block; content: ""; clear: both; } B: .bigbox{clear: both; } C: .bigbox::before{ display: block; clear: both; } D: .bigbox{ display: block; content: "" }