The Equation Gives the Position S = F(t) of a Body

Multiple Choice

The equation gives the position s = f(t) of a body moving on a coordinate line (s in meters, t in seconds) .
- $s = 8 \sin t - \cos t$
Find the body's acceleration at time $t = \pi / 4 \mathrm { sec }$ .

A) $\frac { 7 \sqrt { 2 } } { 2 } \mathrm {~m} / \mathrm { sec } ^ { 2 }$
B) $- \frac { 9 \sqrt { 2 } } { 2 } \mathrm {~m} / \mathrm { sec } ^ { 2 }$
C) $- \frac { 7 \sqrt { 2 } } { 2 } \mathrm {~m} / \mathrm { sec } ^ { 2 }$
D) $\frac { 9 \sqrt { 2 } } { 2 } \mathrm {~m} / \mathrm { sec } ^ { 2 }$