题目

Five years ago, David Smith wore an expensive suit to work every day. “I was a clothes addict,” jokes. “I used to carry a fresh suit to work with me so I could change if my clothes got wrinkled.” Today David wears casual clothes---khaki pants and sports shirt---to the office. He hardly ever wears necktie. “I’m working harder than ever,” David says, “and I need to feel comfortable.” More and more companies are allowing their office workers to wear casual clothes to work in the United States. The change from formal to casual office wear has been gradual. In the early 1990s, man曲线f（x）=xlnx+2x在点（1，f（1））处的切线方程是（　　）A．3x-y-1=0B．3x-y+1=0C．3x+y-1=0D．3x-y-5=0