Which of the following statements holds true for the term plug$-$in$?$

a$.$

It refers to a small computer program that extends the feature set or capabilities of another application.

b$.$

It refers to the generation of bogus clicks, either for financial gain or to attack rivals by draining their online ad budget.

c$.$

It refers to a program that enrolls all customers by default, but that allows consumers to discontinue participation if they want to$.$

d$.$

It refers to recruiting a network of users with the goal of spreading IP addresses across several systems.

e$.$

It refers to a program $($typically a marketing effort$)$ that requires customer consent.