Original sentences: sir edmund was anxious to explore antarctica. so was his son, cecil. combined sentence: sir edmund and his son, cecil, were anxious to explore antarctica. does the combined sentence use a compound subject, compound verb, or compound object?

The combined sentence uses a compound subject.

Explanation

The subject of a sentence is said to be the protagonist.  

It maybe a person, place, or idea.  compound subject using sentences has multiple subjects (two or more) joined by 'and' or 'or'.

When using multiple subjects, the use of subject-verb agreement is important to express the proper idea of the sentence.

Here,  Sir Edmund and Cecil are subjects.

As there are two subjects, this is a sentence that uses two subjects, hence it is a compound subject.

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