单选题 以下程序运行结果是( ). class FatherX{ public Integer getX() { return new Integer(10); }}class Son extends FatherX{ public Double getX() { return new Double(20); }}public class Demo12 { public static void main(String[] args) { FatherX f=new FatherX(); Son s=new Son(); System.out.println(f.getX()+s.getX()); }}