What is a "black hat SEO"?

a$.$

Using unauthorized $($and Google$-$prohibited$)$ techniques to improve a page's search results

b$.$

Paying off a search engine firm's engineers to lower your ad rate

c$.$

Linking to competitor websites in hopes that they will fall in search engine rankings

d$.$

Attempting to hack into a firm's email to manipulate SLA contracts

e$.$

All of the above are true.