An open circuit is very useful, when you want it and you create it intentionally. But an open circuit is a problem when you don't want it and it happens by itself.
An open circuit is a circuit with a break in it, so that current can't flow.
It's very useful when you want to turn stuff off, like lights or the dishwasher.
Then, you use a SWITCH to create an open circuit.
But if, say, a wire breaks or an electrical connection comes apart, and
THAT's what causes an open circuit, then the things plugged into that
circuit don't turn on. It prevents the circuit from working. (B)