Given a curve $y = 7x - x^3$ and $x$ increases at the rate of $2$ units per second. The rate at which the slope of the curve is changing, when $x = 5$ is : (A) $-60 \text{ units/sec}$ (B) $60 \text{ units/sec}$ (C) $-70 \text{ units/sec}$ (D) $-140 \text{ units/sec}$