C. 既有下凸又有上凸                       D. 直线段

19. 函数 y=|lnx| 的拐点是 (         )

A.(1,0)                          B.(e,1)                          C.(2,ln2)                       D. 不存在

20. 函数 y= 的水平渐近线方程是 (         )

A.y=0                           B.y=1                            C.y=3                           D. 不存在

21. , 则 f(x)=(       )

A.3                          B.9                           C. +C                      D.

22. =(         )

A.a^xlna+C                      B.                C.                    D.a^x+lna+C

23. (         )