Vollmer and bourret’s (2000) discovery that the proportion of two- versus three-point shots attempted by basketball players matches the proportion of points obtained from such shots suggests that shot-making in basketball is to some extent reinforced on a schedule.
a) vr
b) vi
c) vt
d) drh

VI schedule.

Explanation:

a Variable Interval schedule (VI) is you are conducting an experiment with different variables but all of those variable have to be applied after a stablished average, for example, there is an experiment of a rat pressing a lever with an average of 10 seconds, the first time, the rat will press the lever exactly at 10 seconds, after that, the rat can press it after 30 or even after 2 seconds, as long as the 10 second average isn't lost.

the correct answer is d

answer; gender,race,age,/// (d) all of these;