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.

faulty parallelism is a construction in which two or more parts of a sentence are equivalent in meaning but not grammatically similar in form.

the correct version of this faulty parallelism in the sentence is:

the hikers knew that the hike was six miles, that it usually required the entire day, and it would not be wise to start after 10: 00 a.m.

the problem is the last sentence, the first two have a basic subject+verb+object structure.

so, to give a proper balance to the structure, or better said, to give a parallel structure, we must write a basic s+v+o structure in all three sentences:

the hikers knew that the hike was six miles, that it usually required the entire day, and it would not be wise to start after 10: 00 a.m.

can i see the painting?