资料题：When Langlon knew that Mark Twain was in love with his sister,（  ）。

Sometimes love can happen by accident.A man known for his mental quickness was able to do something about it.Mark Twain was on board ship, bound for the Holy Land, when he made the acquaintance of a young man named Charles J.Langdon of Elmira, New York.These were the early days of Twain's growing fame.Langdon told Twain how much he admired his writing.Twain was already getting used to this kind of attention, but when Langdon showed him a miniature of his sister,Olivia, he was instantly smitten(打动).He just had to meet the girl.Beck in the United States, Twain wangled(骗取)an invitation to the Langdon home to stay for a week, with the sole intent of finally meeting the beautiful "Livy”, whose face he could not forget.He was not to be disappointed.He was entirely taken with her.He found himself deeply in love, with the week drawing to a close.He knew Livy was the darling of the family, and that his own feelings put him in a di6icult situation wit his hosts.“My week is up," he told Langdon,“and I must go home.”When Langdon pressed to celebrity to stay a bit longer, Twain said,“I ought to go by the first train.I am in love."“In what?"“In love-with your sister, and I ought to get away from here."Much as he admired Twain as a writer, Langdon was instantly alarmed.The consensus in the Langdon family was that no man was good enough for darling Livy.He stopped pressing Twain to extend his stay.En fact he now fully agreed that Twain should leave as soon as possible.Immediately after dinner, they packed into Langdon's wagon to ride to the station, only to be hurtled(急飞)onto the street when the seat of the wagon came loose.Twain was only dazed by the accident, but he made the most of it.He remained at the Langdon house another two weeks to"recuperate." He won the heart of his Livy, and they were later married.Mark Twain had no control over who be would meet on a boat to the Holy Land, and let's hope he wasn't devious enough to loosen the screws on the wagon seat that threw him and Lang-don down to the street.He did, however, have a keen sense for opportunity.For a man deeply in love, it came in handy.