# \begin{align*} \int_{-1}^1 (ax^3 +bx ) dx =0 \end{align*} for :
( A ) $a<0, \; b>0$ only
( B ) $a >0, \; b>0$ only
( C ) any value of $a$ and $b$
( D ) $a >0, \; b<0$ only </span>