Leafage index refers to the ratio of the number of leaves on the main stem of cereal crops to the total number of leaves. It is one of the important indicators to measure the growth process of crops. Based on the above definition, what must be said about the leaf age index? ['Leafage index of rice is directly proportional to its growth process.', 'The larger the leaf age index of buckwheat, the higher its yield.', 'The maximum leaf age index of wheat is 100%.', 'The average leaf age index of corn is greater than that of sorghum.']