A metal rod of Young's modulus $Y$ and coefficient of thermal expansion $a$ is held at its two ends such that its length remains invariant. If its temperature is raised by $t^{\circ} \mathrm{C}$, then the linear stress developed in it is [2011]
(A) $\frac{\alpha t}{Y}$
(B) $Y \alpha t$
(C) $\frac{Y}{\alpha t}$
(D) $\frac{1}{Y \alpha t}$