Which of the Following Statements Holds True for the Term

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

A) It refers to a program (typically a marketing effort) that requires customer consent.
B) It refers to a program that enrolls all customers by default, but that allows consumers to discontinue participation if they want to.
C) It refers to the generation of bogus clicks, either for financial gain or to attack rivals by draining their online ad budget.
D) It refers to a small computer program that extends the feature set or capabilities of another application.
E) It refers to recruiting a network of users with the goal of spreading IP addresses across several systems.