A feed trial to compare three dietary supplements was conducted using 24 pigs of approximately the same body weight. The 24 pigs came from four litters, with each of the four litters containing six pigs. Within a given litter, the six pigs were randomly assigned to the three dietary supplements, with two receiving each supplement. The pigs were housed in 24 identical pens and fed their assigned diets under identical conditions. This is an example of a block design. What are the blocks in this design