A graduate student in agriculture is interested to see how different types of soil will affect the cellulose structure of a certain kind of plant. In the lab, she prepares 8 blocks of soil each with different properties and plants 20 seeds of the plant in each block. After 4 weeks, when the plants are grown enough, she randomly selects 5 plants in each soil type and examines their cellulose structure. What type of sampling did this graduate student use? a. Simple Random Sample
b. Stratified Sampling
c. Cluster Sampling
d. Convenience Sampling