.设f(x)=ax³＋bx²＋cx＋d(a＞0),则f(x)为增函数的充要条件是

A.b²－4ac＞0                                                  B.b＞0,c＞0

C.b=0,c＞0                                                      D.b²－3ac＜0

设f(x)=ax³+x恰有三个单调区间，试确定a的取值范围，并求其单调区间.