A.\(\smallint f\left( x \right)dx = 3{x^3} - x + C\) 
B.\(\smallint f\left( x \right)dx = {x^3} - x + C\)
C.\(\smallint f\left( x \right)dx = \dfrac{1}{3}{x^3} - x + C\)
D.\(\smallint f\left( x \right)dx = {x^3} - C\)