Note that the resistance of the forward biased diode is less than that of the bulb.
 

A and B are the inputs. 
The bulb is the output.
 

Instructions for students
Use a highlighting pen to mark the current flow in each circuit. Use arrows to show the direction of the current flow.
 

Draw up a truth table for this logic gate. Each line of the truth table corresponds to one of these circuits.
 

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