Answer the following question given this paragraph: 

Look at the battery and light bulb in Figure below . The light bulb works and it’s connected to the battery, but it won’t light. The problem is the loose wire on the left. It must be connected to the positive terminal of the battery in order for the bulb to light up. The reason? Electric current can flow through a material such as a wire only if the material forms a closed loop. Charges must have an unbroken path to follow between the positively and negatively charged parts of the voltage source, in this case, the battery.


Q: Because charges must have an unbroken path to follow, electric current cannot flow through a material unless it forms what?


A:
closed loop