An orange grower wishes to compute the average yield from his orchard. The orchard contatins 3 varieties: 50% of his trees are variety A, 25% of variety B, 25% of variety C. Suppose that the grower selects for his sample a 10 by 30 rectangular block of 300 trees of variety A, a 10 by 15 block of 150 B trees, and a 10 by 15 block of 150 C trees. What type of sampling is this?