A metal bar of length L, area of cross-section A, Young's modulus Y and coefficient of linear expansion $\mathrm{\alpha }$, is clamped between two stout  pillars. Now it is heated through t° C. The force exerted by the bar is
 

• A

  $\mathrm{YA\alpha t}$

• B

  Y ALt

• C

  $\mathrm{YA\alpha Lt}$

• D

  $\mathrm{Y}\propto \mathrm{Lt}/\mathrm{A}$