Which Situation Is an Example of Longitudinal Research

Which situation is an example of longitudinal research?

A) A researcher compares how many different age groups perform on a memory test.
B) A researcher compares how the same group of people, at different ages, perform on a memory test.
C) An investigator compares the performance of an experimental group and a control group of participants on a memory test.
D) A researcher compares the performance of several different age groups on a test of memory as each group is tested repeatedly over a period of years.