Solve the Problem $t$ , the Position of a Body Moving Along the S-Axis

Multiple Choice

Solve the problem.
-At time $t$ , the position of a body moving along the s-axis is $s = t ^ { 3 } - 21 t ^ { 2 } + 120 t m$ . Find the body's acceleration each time the velocity is zero.

A) $a ( 10 ) = 0 \mathrm {~m} / \mathrm { sec } ^ { 2 } , a ( 4 ) = 0 \mathrm {~m} / \mathrm { sec } ^ { 2 }$
B) $a ( 10 ) = - 18 \mathrm {~m} / \mathrm { sec } ^ { 2 } , a ( 4 ) = 18 \mathrm {~m} / \mathrm { sec } ^ { 2 }$
C) $a ( 20 ) = 120 \mathrm {~m} / \mathrm { sec } ^ { 2 } , a ( 8 ) = 20 \mathrm {~m} / \mathrm { sec } ^ { 2 }$
D) $a ( 10 ) = 18 \mathrm {~m} / \mathrm { sec } ^ { 2 } , a ( 4 ) = - 18 \mathrm {~m} / \mathrm { sec } ^ { 2 }$