According to the federal regulations, which of the following studies meets the definition of research with human subjects?

a. A feasibility study for implementing a year-round school program, focusing on economic issues such a facilities utilization and transportation costs.
b. The collection of data, by a playground designer hired by the superintendent of schools, about the physical dimensions of school playgrounds, presence of fencing, and the kinds of equipment currently provided.
c. An analysis of aggregate data comparing statewide high school graduation rates provided by the State Department of Public Instruction, using county tax information.
d. A study of twenty 4th grade classrooms in which researchers ask the schools to systematically vary the time of day reading is taught, and collect weekly assessments of reading comprehension for each child over a three-month period.
e. A study of twenty 4th grade classrooms in which researchers ask the schools to systematically vary the time of day reading is taught, and collect weekly assessments of reading comprehension for each child over a three-month period.