After the post-disaster reconstruction, Xiaoyu's permanent residence has been built. Before the whole family moves in, Xiaoyu is going to check whether there is anything wrong with the wires at home.

The answer is B. If the main switch is turned off without turning on any household appliances, the light bulb will glow normally, indicating that the line is closed because the live wire and the neutral wire contact somewhere, that is, short circuit. Imagine that if a light bulb that can withstand 220V is replaced by a fuse that can only withstand a small voltage, the fuse will be blown immediately.