A pendulum clock runs 10 seconds slow per day when room temperature is ${30}^{\circ }\mathrm{C}.$ It runs 20 seconds fast per day when room temperature is ${15}^{\circ }\mathrm{C}.$ The temperature at which clock shows correct time is 

• A

  ${25}^{\circ }\mathrm{C}$

• B

  ${22.5}^{\circ }\mathrm{C}$

• C

  ${20}^{\circ }\mathrm{C}$

• D

  ${35}^{\circ }\mathrm{C}$