问答题
                                            
阅读和理解下面程序段:
  class Manager extends Employee
   public Manager(String n,double s,int year,int month,int day)
    super(n,S,year,month,day);
    bonus=0;
   public double getSalary()
    double baseSalary=super.gerSalary();
    return baseSalary+bonus;
   public void setBonus(double B) bonus=b; 
   private double bonus;
  Manager是Employee的子类,其理由是(  )。