A researcher is examining preschoolers' knowledge of number identification. Fifty preschoolers are grouped based on socioeconomic status (low, moderate, high). Within each SES group, students are randomly assigned to one of two treatment groups: one which incorporates numbers through individual, small group, and whole group work with manipulatives, music, and art; and a second which incorporates numbers through whole group study only. Which of the following methods of blocking is employed here?
a. Predefined value blocking
b. Predefined range blocking
c. Sampled value blocking
d. Sampled range blocking