They gave us a detailed account of what was going on in the remote villages. Which of the following can replace the underlined word?  [    ]

A. expression    

B. description    

C. conversation  

D. statement

已知f（x）=ax³-x²+bx+c，（a，b，c∈R且a≠0）在（-∞，0）上是增函数，在[0，3]上是减函数，且方程f（x）=0有三个实根．

（1）求b的值；

（2）求实数a的取值范围．