以下抽象类的定义中,错误的是 A: abstract class Demo1{} B: abstract class Demo2{<br>public abstract String fun(String a);<br>} C: abstract class Demo3{<br>public String fun(){<br>return "dlvtc";<br>}<br>} D: abstract class Demo4{<br>public static final String str= "dlvtc";<br>abstract void fun(){}<br>}