An immersion heater of 100 W can raise the temperature of 1 liter of water from \( 30^{\circ} \mathrm{C} \) to \( 40^{\circ} \mathrm{C} \) in 7 minutes. Determine the value of the mechanical equivalent of heat.

A \( 4.2 \mathrm{~J} \mathrm{cal}^{-1} \)

B \( 4.3 \mathrm{~J} \mathrm{cal}^{-1} \)

C \( 4.4 \mathrm{~J} \mathrm{cal}^{-1} \)