Handpicking useful in case you need to remove stones from 500 kg rice.

Though, handpicking is the method to remove slightly larger impurities that are easy to pick by hand. But, to use this method, the quantity of the substance to be cleaned, and the number of impurities should be less, so that it is manageable by a person. Now, if the quantity is 500 kg, that is too much for a person to handle, and removing stones from 500 kg is very time-consuming and tedious work. Thus, handpicking stones from 500 kg of rice is not a useful method.