阅读理解。 Mark Twain was a famous American writer. His real name was Samuel Langhorne Clemens. He grew up in a small town beside the Mississippi River around the middle of the nineteenth century. He liked to watch the steamboats (轮船) traveling along the river. When he was seventeen, he went east to New York, but he never forgot the boats on the river in his hometown. At the age of twenty-one he returned home and became a steamboat pilot (领航员). He was very happy on the river. Later, when he began to write stories, he used the name Mark Twain. Mark Twain was a term used by riverboat men. Most people have forgotten that his name was really Samuel Langhorne Clemens. Mark Twain wrote many famous books, but he is remembered most for his stories about young boys. One of his best stories tells about Tom Sawyer. 根据短文内容，判断正（T）误（F）。 (     )1. Mark Twain was born in the USA. (     )2. Mark Twain loved the Mississippi River and his hometown. (     )3. When Mark Twain was seventeen, he left for New York. (     )4. Mark Twain's real name was Tom Sawyer. (     )5. Mark Twain began to write when he was 20.