2f(x)=3x^2-4x+c =3(x^2-4x/3 +4/9)+c-4/3 =3(x-2/3)^2+c-4/3对称轴x=2/3∈[0,5]∵3>0∴当x=2/3时得最小值-4/3+c|2/3-0|=2/3|5-2/3|=13/3 >2/3∴最大值是f(5)=55+c∴选C
