There is a difference between the term social networking and the purpose of social network sites, as described in an article by Boyd and Ellison. Which part of the text passage is a signal phrase?

a
Is a difference
b
The purpose of
c
As described in
d
Boyd and Ellison

C). "As described in"

when a quotation, summary or paraphrase are used this can use a signal phrase and requires a verb, depending on how it was written may also include an name or an object for it to be a proper phrase

search it up on google or think from your brain make it work harder.