Airline frequent flyer programs that reward customers with a free flight after every 75,000 miles of travel illustrate the use of a schedule of reinforcement.

my good sir, your answer here is "jvhrwekjhvbebebjb"

your welcome.

d. i assume the underlined word is "role." reformat if it isn't.

explanation:

role is closest to part because when replacing words, "playing a vital decision," "playing a vital activity," and "playing a vital dollar" don't make logical sense. the only meaningful choice is "playing a vital part."

this fits one of merriam-webster's definitions of role: "a function or part performed especially in a particular operation or process."