What Is Meant by a Longitudinal Design?​

What is meant by a longitudinal design?​

A) A study of several ages in several cultures, at either the same or different times​
B) ​A study of one group of people at several times as they grow older
C) ​A study of people differing in age, at one point in time
D) ​A study of one group of people to different people of a different age at a different time