A bird is flying at a speed of $5 \mathrm{~m} / \mathrm{s}$ in the direction of the vector $4 \hat{i}+4 \hat{j}-2 \hat{k}$. The temperature of the region is given by $T=x^{2}+y^{2}-z^{2}$
The rate of increase of temperature per unit time, at the instant it passes through the point $(1,1,2)$ is :
(a) $\frac{60}{3}{ }^{\circ} \mathrm{C} / \mathrm{s}$
(b) $3^{\circ} \mathrm{C} / \mathrm{s}$
(c) $18^{\circ} \mathrm{C} / \mathrm{s}$
(d) $4^{\circ} \mathrm{C} / \mathrm{s}$