Read the sentences: She took off the backpack and opened it. Inside were three textbooks and a full binder.

Write down which sentence contains a compound subject.

1. the relationship between a pronoun and its antecedent- reference

reference refers to the relationship between mainly pronoun and another unit of grammar- noun phrase or noun.

2. a singular noun naming a collection or group- collective noun.

collective noun refers to the kind of noun which refer to the name of people, thing or anything in collective. for instance bundle of sticks, herein 'bundle' is collective noun.  

3. a modifying clause whose subject and/or verb is implied- elliptical clause

it is a clause in which words are being left out. on the other hand, the missing word is understood due to the logic or pattern of the sentence.  

4. two or more nouns and/or pronouns serving the same grammatical function and followed by a pronoun that references one or all nouns- compound antecedent

compound antecedent refers to which is made up of two or more words which are joined by 'or' or 'and.'

5. an adjective clause introduced by a pronoun- relative clause.

relative clauses are the clauses which begin with the relative pronouns such as when, where, which, that or who. the noun that precedes, relative pronouns identify or define that noun.  

6. the word, phrase, or clause to which a pronoun refers- antecedent.

antecedents refers to the phrases, clauses or words which are replaced with one of the third personal pronouns.