A particle is moving in a straight line. The variation of position ‘\( x \)’ as a function of time ‘\( t \)’ is given as \( x = (t^{3} - 6 t^{2} - 20 t + 15 )\, \text{m} \). The velocity of the body when its acceleration becomes zero is:
\( 4 \, \text{m} / \text{s} \)
\( 6 \, \text{m} / \text{s} \)
\( 8 \, \text{m} / \text{s} \)
\( 10 \, \text{m} / \text{s} \)
