Most analysts focus on the cost of tuition as the way to measure the cost of a college education. But incidentals, such as textbook costs, are rarely considered. A researcher at Drummand University wishes to estimate the textbook costs of first-year students at Drummand. To do so, she monitored the textbook cost of 250 first-year students and found that their average textbook cost was $300 per semester. Identify the variable of interest to the researcher.

a. The textbook cost of first-year Drummand University students.

b. The year in school of Drummand University students.

c. The age of Drummand University students.

d. The cost of incidental expenses of Drummand University students.