HTML5中doctype标签的正确定义是（ ）
A<!DOCTYPE html PUBLIC "-//W3C//DTD// XHTML 1.0 Transitional//EN" "http://w3.org/TR/xhtml1-transitional.dtd">
B<!DOCTYPE html PUBLIC "-//W3C//DTD// XHTML 1.0 Transitional//EN" "http://w3.org/TR/xhtml4/loose.dtd">
C<!doctype html5>
D<!doctype html>