听力原文： (16) Daniel Hale Williams was an example of a brilliant African-American doctor who managed to find success despite the obstacles white Americans placed in front of him. He was born in 1856 in Hollidaysburg, Pennsylvania. Almost immediately after graduating, (17) he needed to open his own office because at that time no Chicago hospitals would allow black doctors to 'use modern facilities and equipment to complete their medical training and care for patients'. After two years of successfully running his own hospital, Daniel Hale Williams was confronted with an operation that had never been accomplished. While working in the emergency room, a black express man, James Cornish, rushed in with a knife trauma into his chest. (18) None of the other doctors seemed to know what to do since the external bleeding had stopped, plus there was no way for them to see what was happening inside his chest. This was because there were no x-ray machines available to the doctors, because they were not yet discovered. As the night progressed, Cornish's state began to diminish. After careful evaluation and assessment, Daniel decided that performing open heart surgery on Cornish was the most promising procedure for them to take. Daniel took on the challenge of performing an operation without much technology or advancement. While in the operating room, Daniel 'decided to operate on the left side of the chest cavity. (19) The surgery was a success and James Cornish lived for twenty years after the operation was completed. With all of this agony and strength, Daniel Hale Williams became the first person to perform. a successful heart surgery.' 16. What is the purpose of the passage? 17.Why didn't Williams work at a hospital? 18.Why hadn't open heart surgery been performed before? 19.How do we know the operation was really successful? (7)