Sentences with many clauses and phrases are difficult to understand because the clauses and phrases typically change the intended meaning of the sentence refer to other sentences in the passage modify other clauses and phrases in the sentence make it hard to determine where the sentence ends.

The correct answer is: modify other clauses and phrases in the sentence

Explanation:

Clauses and phrases must always be followed or preceded by an independent clause in order to formulate a complete sentence and one complete thought. Various clauses and phrases can be confusing, making it difficult for readers to tell which independent clause they modify.