Which of the following best describes vertical integration?

Question $2$ options:

a$)$

to build long$-$term partnerships with a few suppliers

b$)$

to develop the ability to produce products which complement the original product

c$)$

to sell products to a supplier or a distributor

d$)$

to produce goods or services previously purchased

e$)$

to develop the ability to produce the specified good more efficiently than before