Please help me! Giving right answer brainiest!

A group of eight people work on the fifth floor of the Peyton Building. They all decide they want to lose weight by getting more exercise. Four of them decide to walk up the four flights of stairs each morning. The other four decide to walk down the four flights of stairs at the end of the day. They weigh themselves each Friday morning and compare. This is an example of
(1 point)
A.an observational study, since they were not assigned to their groups.
B.an experiment, since they are randomly divided into two equal groups.
C.block design, since there are two treatments.
D.a survey, since the people, volunteered to measure their weight each week.
E.an explanatory variable, since they both increase their exercise.