Check all that is true about the function f(x)=x4 - 6x3 : f has critical numbers at x=0 and x=9/2 f is increasing for x>9/2 and has a local minimum at x= 9/2 f is concave up on the interval (0, 3) and has inflection points at x=0 and x=3. f is concave up for x<0 and for x>3; has inflection points at x=0 and x=3.