Why Is a Longitudinal Design Considered a Within-Subjects Design

Why is a longitudinal design considered a within-subjects design?

A) Participants are measured twice, once at the beginning of the study and once at the end of the study.
B) Participants are repeatedly measured on the dependent variable over time.
C) Participants are measured on the dependent variable after exposure to each level of the independent variable.
D) Fewer research participants are needed for adequate statistical power.