已定义类A: class A{ private int x = 1; protected int y = 2; public int z = 3; public int sumA(){return(x + y + z); } } 再通过继承与扩展定义子类B: class B extends A{ private int b = 4; public int sumB(){ int s = 0; s += x; s += y; s += z; s += b; return s; } } 方法成员sumB()中错误的语句是( )。