Grimstvedt and colleagues (2010) examined the effect of placing signs encouraging the use of stairs near elevators. Based on their design, Liu Wei designs a study for his campus. He selects four buildings and starts by measuring elevator and stair use in those buildings for a month. He then posts signs encouraging the use of stairs near the elevators and measures elevator and stair use in the buildings for another month. Identify the true and false statements about Liu Wei's study.

a. Liu Wei is utilizing a stable-baseline design.
b. Liu Wei's study involves a within-groups design.
c. Liu Wei's study design has high external validity.
d. Liu Wei's study is a quasi-experiment.