设方程xlny＝ylnx确定隐函数y＝f(x)，则在x=1处的微分dy＝( )。
A．-1
B．-dx
C．1
D．dx
参考答案：D
_____ is reported that Liu Xiang won a gold medal in the Asian Games in Guangzhou. [ ]
A. It
B. As
C. What
D. Which
耐磨材料属于非均匀性的（）材料。
A.磁性
B.抗氧性
C.脆性
D.活性炭
