In the Gold Rush of 1849, some one hundred thousand immigrants were lured to California by the promise of gold. Towns and businesses popped up to accommodate the growing population, but a problem remained: how to communicate across the vast reaches of the United States. Three businessmen thought they had an faster mail delivery via the Pony Express, a relay system of horses and riders. The Pony Express made its first delivery on April 14, 1860. Mail from St. Joseph, Missouri, reached San Francisco in just eleven days, rather than a month or more, which was standard for mail delivery at the time. The Pony Express was a short-lived solution, however. After just eighteen months, it was replaced by a national telegraph wire. What is the main idea of the passage?

The main idea of the text is to present how the Pony Express made communication in the US easier at a time when the country was experiencing a high growth rate.

Explanation:

The main idea of a text is the main subject that this text addresses and that will govern all the information presented during the text. In the text above, we can see that the main idea is the fact that Pony Express streamlined communication in the US and how this was very beneficial for the country's growth. That's because in the text, we can see that all the information turns on how the creation of Pony Express allowed the correspondence to be delivered much faster.

the answer is:

when will (v) the books (n) be (v) due?

nouns refer to people (other than pronouns), places and things, whereas verbs describe actions and are usually part of the predicate of a sentence. finally, pronouns can work as nouns and identify the participants in speech.

in the example sentence, "will" is a verb part or auxiliary verb, "be" is an infinitive verb and "books" is a plural noun.