What is a thermostat used for?

The area of cross-section of a railway track is $0.01{ \mathrm{m}}^2.$ The temperature variation is $10 °\mathrm{C}.$ Coefficient of linear expansion of material of track is ${10}^{-5} {}^{\circ }\mathrm{C}^{-1}.$ The energy stored per meter in the track is $\mathrm{J} {\mathrm{m}}^{-1}.$ (Young's modulus of material of track is ${10}^{11} \mathrm{N} {\mathrm{m}}^{-2}$)

Rivet is heated before fixing in the hole to join two metal plates.

At what temperature is the density of water maximum? 

$$\begin{gathered} \mathrm{A}: {100}^{\circ }\mathrm{C} \\ \mathrm{B} : 0^{\circ }\mathrm{C} \\ \mathrm{C} : 4^{\circ }\mathrm{C} \end{gathered}$$

Enter your correct as $\mathrm{A}, \mathrm{B} \mathrm{or} \mathrm{C}$.

Borosilicate glass does not expand much on being heated.