Which of the following is a longitudinal study?

a. Researchers test the intelligence of all the students in a high school.

b. Intelligence tests are given to the residents of a nursing home.

c. Researchers randomly select 50 students from a high school with 2000 students. The 50 students are given intelligence tests.

d. A group of college juniors is given an extensive battery of tests over a period of 2 days.

e. A group of kindergartners is given an intelligence test. They are retested every other year for 30 years.