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( )。