任务描述：建立的Truck类，扩展Car类，并覆盖speedUp()方法，使速度累加50、

<?php

class Car {

    public $speed = 0; //汽车的起始速度是0

    public function speedUp() {

        $this->speed += 10;

        return $this->speed;

    }

}

//定义继承于Car的Truck类

class Truck extends Car{

    public function speedUp(){

        $this->speed = parent::speedUp() + 50;

        return $this->speed;

    }

}

$car = new Truck();

$car->speedUp();

$car->speedUp();

echo $car->speed;

如果调用两次speedUp()方法，输出：120。这还是Truck类的speed累加50么？