Volunteers for a Human Performance Study Were Randomly Divided into Two

Multiple Choice

Volunteers for a human performance study were randomly divided into two groups. The first group had their flexibility measured in the morning after a short meditation session while the second group had their flexibility measured in the afternoon with no previous meditation session. The flexibility scores of the two groups were compared.
In this study, the explanatory variable is:

A) the volunteer's flexibility score.
B) whether the volunteer had a meditation session.
C) the placebo.
D) a lurking variable.