You work at Sunny Hills Garden and Nursery and are given this document. The rationale behind providing several different suggestions for barriers is:
O to educate the customer on which options to avoid.
O to maximize the sales options given to the customer.
O to discuss the benefits and drawbacks of each method.
O to prevent the customer from using a single impediment.
O to encourage a specific method for blocking weed growth.

answer; researchers typically cannot control people's sexual behaviors as would be necessary in an experiment;

when your a dog and you need a bone.   go and eat a rabbit

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."