free residual chlorine

例句：

1. 1. The free samples were a great way to attract customers.
   免费样品是吸引顾客的好方法。
2. 2. We can't afford the expensive car;it's free for us.
   我们买不起那辆昂贵的车，它对我们免费。
3. 3. The company offers free shipping on orders over $50.
   该公司对订单超过50美元的部分提供免费快递服务。