Read the sentences. Grapefruits are citrus fruits. Lemons and oranges are also citrus fruits.
Which combines the sentences by using a compound subject?
Grapefruits, lemons, and oranges are citrus fruits.
Grapefruits and lemons are citrus fruits, and so are oranges.
Grapefruits are citrus fruits, and lemons and oranges are, too.
Grapefruits, in addition to lemons and oranges, are citrus fruits.

Grapefruits, in addition to lemons and oranges, are citrus fruits.

Explanation:

All of these may work to some extent, but on account that this sentence is the only one that sounds completely right, I believe the last choice is correct.

Sentences can become compound by using words like "but, and, so, in addition, etc.", and all of these sentences are found to be using these kinds of   verbs. A compound sentence is a sentence that should be using at least one verb to combine two or more sentences.

d) the narrator even suggested that the white whale, moby d, was ubiquitous and present in every ocean at every moment.  

- "a nonrestrictive clause functions as a modifier to a noun (that is an object, person or concept). these clauses can be described by the following three options: it is set off by comas, it contains a subject and a verbs and it adds nonessential information to the sentence.

for example, in "here is bill gates, who confounded microsoft software company" we can identify "who confounded microsoft software company" as a nonrestrictive clause modifying the noun bill gates. this clause can be describes as a nonrestrictive clause because it is separated by a comma, it adds nonessential information to identify who bill gates is and it must contain both a subject (who) and a verb (confounded)."