In the activities to demonstrate the heating effects of electric current, why isn't the switch turned on for very long durations?

According to Joule's law of heating heat produced is given by

$Heat=I^{2}Rt$ where $t$ is time for which circuit was on. So more time means more heat developed hence wires may become very hot to touch.

The heating effect of electric current causes the cell to turn weak quickly if turned on for long durations .Hence options $A$ & $B$ are correct.