A、It is the name of a riverboat. B、It is a pen name of a famous writer. C、It is a term of communication among pilots. D、It is a

The writer who became Mark Twain was born as Samuel Langhorne Clemens in 1835. After his father died in 1847, young Samuel went to work as an assistant to a publisher. Ten years later, he became a pilot on a steamboat that sailed on the Mississippi.(23)He heard the riverboat workers call out the words "mark twain," which was a measure for the depth of water. Later, he wrote funny stories and called himself Mark Twain. Twain also traveled a lot and began writing books about his travels. His stories about a trip to Europe and the Middle East were published in The Innocents Abroad. And his stories about life in the western United States became the book called Roughing It. After he got married, he wrote books for children, such as The Prince and the Pauper. It tells about a poor boy who trades identities with a member of England’s ruling family.(24)Twain also wrote Life on the Mississippi, which describes his days as a steamboat pilot and his return to the river 20 years later. Mark Twain was already a successful writer before he became famous as a public speaker. Over the years, he had invested a lot of money in unsuccessful businesses. In 1893, he found himself deeply in debt.(25)To earn money, he traveled around the world giving humorous talks. His speeches made people laugh and remember those events they had experienced.
23. What does "mark twain" mean originally?
24. What does Life on the Mississippi describe?
25. Why did Mark Twin give speeches?