If `f(x)=((a^(2)-1)/(3))x^(3)+(a-1)x^(2)+2x+1` is monotonically increasing for every `x in R` then `a` can lie in
(A) `(1,2)`
(B) `(1,oo)`
(C) `(-oo,-3)`
(D) `(-10,-7)`