Suppose $a < b$. The maximum value of the integral
$$\int _ { a } ^ { b } \left( \frac { 3 } { 4 } - x - x ^ { 2 } \right) d x$$
over all possible values of $a$ and $b$ is\\
(A) $\frac { 3 } { 4 }$\\
(B) $\frac { 4 } { 3 }$\\
(C) $\frac { 3 } { 2 }$\\
(D) $\frac { 2 } { 3 }$