According to the federal regulations, which of the following

definition of research with human subjects?

A feasibility study for implementing a year$-$round school program, focusing on economic issues such as facilities utilization and transportation costs.

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.

A study of twenty $4$th 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.

An analysis of aggregate data comparing statewide high school graduation rates provided by the State Department of Public Instruction, using county tax information.