The heat developed in an electric wire of resistance $R$ by a current $I$ for a time $t$ is

• A
  $\frac{I^2 Rt}{4.2} \text{ cal}$
• B
  $\frac{I^2 t}{4.2 R} \text{ cal}$
• C
  $\frac{I^2 R}{4.2 t} \text{ cal}$
• D
  $\frac{Rt}{4.2 I^2} \text{ cal}$