The relation between time \( t \) and distance \( x \) for a moving body is given as \( t = m x^2 + n x \), where \( m \) and \( n \) are constants. The retardation of the motion is: (Where \( v \) stands for velocity)
\( 2 m v^3 \)
\( 2 n v^3 \)
\( 2 n^2 v^3 \)
\( 2 m n v^3 \)