Which of the following correctly summarizes the main point of the introductory paragraph?

the pony express was the first mail delivery system that reached the west coast.
the pony express delivered mail in ten days or less to most places.
the pony express crossed the continent from missouri to the pacific.
the pony express delivered messages on horseback through many areas.

type of error:

the error in the sentence is a "dangling modifier". a dangling modifier is a phrase, word or clause that "dangles" because there is not target to modify. in this case, "after mopping the floor" is the dangling modifier since it has no word to modify. the noun "house" cannot mop the floor; therefore there is a mistake in the writing of the sentence.

can you take a picture of the passage so i can answer your question