One of the well-known of American writers is Samuel Clemens, whose pen name is Mark Twain. Born in 1835, Twain grew up in the Mississippi River town of Hannibal, Missouri. As did many other boys of his day, Twain dreamed of traveling on river boats and of someday becoming a riverboat pilot. Twain used his memories of the life of a river town in his two most famous books, Huckleberry Finn and Tom Sawyer. As a young man, Twain held many jobs. He was a printer, a good miner, and, for a time, he was a riverboat pilot. During his pilot days, he adopted the name Mark Twain. This was a term used by the boatmen to mean that the water measured two fathoms, or twelve feet, which was deep enough for safe passage. Finally Twain became a successful writer. He traveled a great deal, writing and speaking, and became very popular both in the United States and in Europe. Twain’s style of writing was and direct. Among the things he wrote about were superstitious (迷信的) people and people who were easily fooled. He used his unusual gift for humor to write about many things of importance.

Where did the name Mark Twain come from（）

A．It came from a safe passage.
B．It came from a term used by the boatmen.
C．It came from his boyhood .
D．It came from his measuring tool.