Some people think that the concentration of milk has something to do with the organic pastures that cows eat. In order to verify this point, which of the following experimental methods is the most reliable? ['Select dairy cows of the good breed as experimental samples.', 'Laboratory comparison of the nutrient composition of organic forage and fertilizer forage.', 'Choose two batches of cows of the same breed and level as far as possible, one batch is fed with organic pastures, and the other batch is fed with fertilized pastures. After a period of time, the concentration of milk produced by the two batches is compared.', 'For the same batch of cows, gradually reduce or increase the proportion of organic forage in their food, and compare the concentration of milk produced by this batch of cows in different periods.']